Si Non Credideritis, Non Intellegetis (cf. Isaiah 7:9 Septuagint rendering)

UNLESS YOU BELIEVE YOU WILL NOT UNDERSTAND

That is the title of Chapter Two (nos. 23-36) of the last papal encyclical--Lumen Fidei--which was mostly written by Pope Emeritus Benedict and promulgated by Pope Francis 29 June 2013. After dealing with the problem of the translation, which has been mentioned elsewhere, He gives the following explanation for why faith and truth are inseparable.

"We need knowledge, we need truth, because without these we cannot stand firm, we cannot move forward. Faith without truth does not save, it does not provide a sure footing. It remains a beautiful story, the projection of our deep yearning for happiness, something capable of satisfying us to the extent that we are willing to deceive ourselves...But precisely because of its intrinsic link to truth, faith is instead able to offer a new light, superior to the king's calculations, for it sees further into the distance and takes into account the hand of God, who remains faithful to his covenant and his promises.

"Today more than ever, we need to be reminded of this bond between faith and truth, given the crisis of truth in our age. In contemporary culture, we often tend to consider the only real truth to be that of technology: truth is what we succeed in building and measuring by our scientific-know-how; truth is what works and what makes life easier and more comfortable...[the same kind of truth of the great destructive] totalitarian movements of the last century, a truth that imposed its own worldview in order to crush the actual lives of individuals. In the end, what we are left with is relativism, in which the question of God--is no longer relevant."

The encyclical goes on to show the necessary connection between faith and love and faith and divine revelation.

In his Commentary to the De Trinitate of Boethius, Saint Thomas Aquinas masterfully and systematically treated the same question (using the same scripture quote [at the end of the "response" below]) clearly distinguishing between the truth of faith and mere opinion. Here is the relevant question (Question III, Article 1).

QUESTION III Concerning Those Things That Pertain to the Knowledge Possessed by Faith

Article 1
Whether Faith Is Necessary for Mankind

Objections

1. It seems that faith should not be considered necessary for mankind. As is said in Eccles. 7:1, “Why does a man need to seek things that are above him?” This is to say, there is no need. But those things that are believed by faith are above man, as exceeding his reason; otherwise his reason, which is the cause of science, would suffice. Therefore it was not necessary for man that, over and above the truths of reason, he should be taught those of faith. 2. God established human nature as something perfect when He created it. Deut. 32:4, “The works of God are perfect.” But from the ability bestowed upon the human mind according to its original condition, man cannot attain to those things which must be known by faith; otherwise he would be able to possess scientific knowledge of them, a knowledge which is caused by the fact that conclusions are resolved into naturally known principles. Since, therefore, a thing is called perfect if it lacks nothing that it ought to possess, as is said in V Metaph., it seems that man does not require faith. 3. Every wise man makes choice of the shorter way to reach a goal: but it would appear exceedingly difficult for a creature to believe truths which are above reason and, in the case of men, extremely dangerous, since many fall away from the state of salvation because they do not believe; therefore, it seems that God, who is all-wise, ought not to have established faith as the way of salvation for men.

4. Whenever there is acceptance of knowledge without judgment, the road to error is easy; but we have in ourselves no ability by which we are able to judge of the things which we accept by faith, since our natural judgment does not extend to truths of this kind, as they exceed reason; therefore evidently the road to error is an easy one for us, and so it would appear rather harmful than useful for man that he should be directed to God by the way of faith.

5. As Dionysius says, it is an evil for man to exist apart from reason; but man in adhering to faith departs from reason, and in this he is even accustomed to despise reason; therefore it seems that such a way is evil for men.

Sed contra

But on the contrary, it is said in Heb. 11:6, “Without faith it is impossible to please God”; but it is supremely necessary for man that he be pleasing to God, since otherwise he can neither do nor possess any good; therefore faith is most necessary for man.

Again, it is most necessary for man to know the truth, since beatitude is joy in knowing the truth, as Augustine says; but faith establishes believers in truth and establishes truth in them, as Dionysius says (De div. nom., chap. 7); therefore faith is most necessary for man.

Again, that without which human society cannot be conserved is especially necessary for man, since man is a political animal, as is said in VIII Ethic.; but without faith human society cannot be preserved, since it is requisite that one man believe in the promises of another and in his testimony and the like, for this is necessary if they are to live together; therefore faith is most necessary for mankind.

Response. I answer that it must be said that faith has something in common with opinion, and something in common with knowledge and understanding, by reason of which it holds a position midway between opinion and understanding or science, according to Hugh of St. Victor. In common with understanding and knowledge, it possesses certain and fixed assent; and in this it differs from opinion, which accepts one of two opposites, though with fear that the other may be true, and on account of this doubt it fluctuates between two contraries. But, in common with opinion, faith is concerned with things that are not naturally possible to our understanding, and in this respect it differs from science and intellection.

That a thing should not be apparent to human understanding can arise for two reasons, as is said in II Metaph.: namely, because of lack of knowability in things themselves, and because of lack of intellectual ability on our part.

1. It may be due to lack on the part of things, as in the case of singular and contingent things which are remote from our senses, like the deeds and words and thoughts of men; for these are of such a nature that they may be known to one man, but unknown to others. And since among men dwelling together one man should deal with another as with himself in what he is not self-sufficient, therefore it is needful that he be able to stand with as much certainty on what another knows, but of which he himself is ignorant, as upon the truths which he himself knows. Hence it is that in human society faith is necessary in order that one man give credence to the words of another, and this is the foundation of justice, as Tullius says in his book, De officiis. Hence also it is that no lie is without sin, since every lie derogates from that faith which is so necessary.

2. The truth of things may also not be evident because of defect on our part, as in the case of divine and necessary things which, according to their own nature, are most knowable. Wherefore, to understand them, we are not capable of immediate intellection, from the very beginning, since it is in accordance with our nature to attain from things less knowable and posterior in themselves, to knowledge of those that are themselves more knowable and prior. But since from none of those things that we know last do we have any knowledge of those that we know first, it is needful for us even at first to have some notion of those things that are most knowable in themselves; but this cannot be except by believing. And this is evident even in the order of the sciences; since that science which is concerned with highest causes, namely, metaphysics, comes last in human knowledge; yet in sciences that are preambles to it there must be supposed certain truths which only in it are more fully revealed; therefore every science has some suppositions that must be believed in order to carry on the process of learning.

Since, therefore, the end of human life is beatitude, which consists in the full cognition of divine truths, it is necessary that human life be directed to this beatitude by an initial possession of divine truths by faith, truths which man can hope to know fully in the ultimate state of human perfection.

Certain of these truths that must be known can be attained by reason even in this life: however, although knowledge of them is possible and even possessed by certain men, nevertheless faith is necessary for five reasons, which Rabbi Moses enumerates:

1. First, on account of the depth and subtlety of the matter, by which divine truths are hidden from human understanding. Therefore, lest any man be without some knowledge of them, provision is made that through faith, at least, he know divine truths. Therefore, in Eccles. 7:25 it is said: “It is a great depth, who shall find it out?” 2. Secondly, on account of the weakness of the human intellect from the beginning. For perfection of knowledge does not belong to the human intellect except at the end; therefore, that it should at no time lack a knowledge of God, it requires faith by which it may accept divine truths from the very beginning. 3. Thirdly, because of the many preambles that are required for a knowledge of God according to reason. For this there is needed knowledge of almost all the sciences, since cognition of divine things is the end of them all. But few indeed would comprehend these preambulatory truths or investigate them completely. Therefore, lest large numbers of men should be left without knowledge of divine things, the way of faith has been provided by God Himself. 4. In the fourth place, many men on account of their natural constitution are unfitted for perfect intellectual investigation according to reason; therefore, that these might not lack knowledge of divine truths, the way of faith has been provided. 5. In the fifth place, because of numerous occupations with which men are busied, it would be impossible for all of them to discover, by way of reason, necessary truth in regard to God, and on this account the way of faith has been established, both as regards things that might in some way be known and as regards those that required revelation in order that they be believed. But in the case of certain divine truths, for a complete understanding of them the human mind in no way suffices, but full knowledge of them is to be awaited in that future life when there will be complete beatitude: such is the truth of the Trinity and the unity of one God; and man is led to knowledge of this, not in accordance with anything due his nature, but by divine grace alone. Therefore it is necessary that, for a perfection of knowledge of this kind, certain suppositions be proposed which must be believed at first, and from these one is directed into full cognition of those truths which at the outset he held on faith, even as in other sciences also, as has been said. Hence in Is. 7:9 it is said, according to one translation: “Unless you believed, you would not understand.” And suppositions of this sort are those that must be believed by all, since in this life they are neither known nor understood by, any one. Answers to objections 1. It may be said: Although matters of faith considered according to man’s natural powers are above him, they are not above man when he is illuminated by divine light; hence it is not necessary for man that he seek out such truths by his own power, but it is necessary for him to know them by divine revelation. 2. It may be said: God, in the first creation of things, established man as perfect in accordance with the perfection of his nature, and this consisted in the fact that man had all things due to his nature. But over and above that due to nature there were added afterward to the human race certain other perfections owing their source to divine grace alone, and among these was faith, as is evident from Eph. 2:8, where it is said of faith that it is “the gift of God.” 3. It may be said: For anyone striving to attain beatitude it is necessary to know in what he ought to seek this beatitude, and in what way. But this, indeed, can be done in no easier way than through faith, since investigation by reason cannot attain to such knowledge except after a previous knowledge of many other things, things not easy to know. Nor can one attain to such knowledge without danger, since human investigation, because of the weakness of our intellect, is prone to error; and this is clearly shown by reference to those philosophers who, in attempting to find out the purpose of human life by way of reason, did not find in themselves the true method, and so fell into many and shameful errors; and so greatly did they differ among themselves that scarcely two or three among them all were in agreement on any one question; yet, on the other hand, we see that by faith many peoples are brought to the acceptance of one common belief. 4. It may be said: Whenever there is acceptance of a truth, by whatever mode of assent, there must be something which moves the mind to assent: just as the naturally possessed light of the intellect causes assent to first principles, and the truth of those first principles causes assent to conclusions made from them; while in other ways we assent to things of which we have an opinion, though, if motives were a little stronger, they would incline us to belief, in so far as faith is said to be opinion. But that which inclines the mind to assent to the first principles of understanding or to conclusions known from these principles is a sufficient induction which forces assent, and is sufficient to judge of those things to which the mind gives its assent. On the other hand, whatever inclines one to form an opinion, even though with a good amount of conviction, is not that sufficient form of induction whereby assent is forced, nor by reason of it can there be perfect judgment of the things to which assent is given. Therefore also in faith by which we believe in God, not only is there acceptance of the truths to which we give assent, but also something which inclines us to that assent; and this is the special light which is the habit of faith, divinely infused into the human mind. This, moreover, is more sufficient for inducing belief than any demonstration, for, though from the latter no false conclusions are reached, still man frequently errs in this: that he thinks something is a demonstration which is not. The light of faith is also more sufficient than the natural light of reason by which we assent to first principles, since this natural light is often impeded by bodily infirmity, as is evident in the case of the. insane. But the light of faith, which is, as it were, a kind of impression of the First Truth in our minds, cannot fail, any more than God can deceive us or lie; therefore this light suffices for making judgment. This habit of faith, nevertheless, does not move us by way of intellectual understanding, but more by way of the will; therefore it does not make us comprehend those truths which we believe, nor does it force assent, but it causes us to assent to them voluntarily. And thus it is evident that faith comes in two ways: namely, from God by reason of the interior light which induces assent, and also by reason of those truths which are proposed exteriorly and take their source from divine revelation. These latter are related to the knowledge which is of faith as things known by the senses are to knowledge of first principles, because in both cases there is a certain determination given to cognition. Therefore, as cognition of first principles is received by way of sense experience, and yet the light by which those principles are known is innate, so faith comes by way of hearing, and yet the habit of faith is infused. 5. It may be said: To live in accordance with reason is the good of man inasmuch as he is man. Now, to live apart from reason, according to one meaning, can be understood as a defect, as it is in those who live according to sense; and this is an evil in man. But in another way, it may mean to live above reason as when, by divine grace, a man is led to that which exceeds reason: and in this case, to live apart from reason is not an evil in man, but a good above that which is human. And such is the cognition of truths of faith, although faith itself is not in every way outside reason; for, it is the natural reason which holds that assent ought to be given to truths declared by God.