Thus the 2011 prologue to his first doctoral thesis on Augustine: Gesammelte Schriften I: Volk und Haus Gottes in Augustins Lehre von der Kirche, Freiburg: Herder, 2011, 8-9.
Below is one of the chapters of that same book ten of The City of God so praised by Ratzinger.
Book x Chapter 6
Of the true and perfect sacrifice
A true sacrifice...is every work done in order that we may draw near to God in holy fellowship: done, that is, with reference to that supreme good and end in which alone we can be truly blessed. Therefore, even the mercy which we extend to men is not a sacrifice if it is not given for God's sake. For, though performed or offered by man, a sacrifice is a divine thing, as the Latin authors of old showed when they used the word sacrificium. Thus, a man who is consecrated in the name of God and pledged to God is himself a sacrifice insofar as he dies to the world so that he may live to God. (Cf. Rom. 6,11) For this too pertains to mercy: to that mercy which each man shows to himself. And so it is written, 'Have mercy on thy soul by pleasing God.' (Ecclus. 30,24) Our body also is a sacrifice when we chasten it by temperance, if we do so, as we ought, for God's sake, so that we may not yield our members as instruments of unrighteousness unto sin, but as instruments of righteousness unto God. (Cf. Rom. 6,16f.) The apostle exhorts us to this when he says: 'I beseech you therefore, brethren, by the mercy of God, that ye present your bodies a living sacrifice, holy, acceptable to God, which is your reasonable service.' (Rom. 12,1) The body, then, which, because it is inferior, the soul uses as a servant or instrument, is a sacrifice when it is used rightly and with reference to God. And, if this is so, how much more does the soul itself become a sacrifice when it directs itself to God so that, inflamed with the fire of His love, it may receive His beauty and be pleasing to Him, losing the form of worldly desire and being reformed immutably by its submission to Him! This, indeed, the apostle adds in what follows, when he says: 'And be not conformed to this world; but be ye transformed in the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect will of God.' (Rom. 12,2)
Since, therefore, true sacrifices are works of mercy shown to ourselves or to our neighbours, and done with reference to God; and since works of mercy have no object other than to set us free from misery and thereby make us blessed; and since this cannot be done other than through that good of which it is said, 'It is good for me to be very near to God': (Psalm 73,28) it surely follows that the whole of the redeemed City--that is, the congregation and fellowship of the saints--is offered to God as a universal sacrifice for us through the great High Priest Who, in His Passion, offered even Himself for us in the form of a servant, so that we might be the body of so great a head. (Cf. Phil. 2,7) For it was this form that he offered, and in it that he was offered, because it is according to it that he is a our Mediator. In this form He is our Priest; in it, He is our sacrifice. Thus, when the apostle has exhorted us to present our bodies as a living sacrifice, holy, acceptable to God, our reasonable service, and not to be conformed to this world, but to be transformed in the renewing of our mind, that we might prove what is that good, and acceptable , and perfect will of God, that is, the true sacrifice of ourselves, he says: 'For I say, through the grace of God which is given unto me, to every man that is among you, not to think of himself more highly than he ought to think, but to deal soberly, according as God hath dealt to every man the measure of faith. For, as we have many members in one body, and all members have not the same office, so we, being many, are one body in Christ, and every one members of one another, having gifts differing according to the grace that is given to us.' (Rom. 12,3) This is the sacrifice of Christians; 'We, being many, are one body in Christ.' And this also, as the faithful know, is the sacrifice which the Church continually celebrates in the sacrament of the altar, by which she demonstrates that she herself is offered in the offering that she makes to God.