That is a colloquial way to summarize today's first reading 1 John 2:22-28 in which Saint John says that anyone who denies Jesus Christ denies God the Father too because Jesus Christ and the Father are one: the one and only living and true God. The denier is a liar: the antichrist!
That is part of what the Lord meant when He said "He who is not with me is against me, and he who does not gather with me scatters." Mt. 12:30
N.B. Recall the two standards of The Spiritual Exercises of Saint Ignatius of Loyola. There are only two armies (two sides, two camps, two companies): one belongs to Christ and the other to Satan. There is no third way! That is why Christ calls Himself "the Way" (and the Truth and the Life). ("I am the way, and the truth, and the life. No one comes to the Father but through me." John 14:6)
You must ensure that you are fighting under the flag of Christ! Everyone must belong to Christ's company!
(Father John Hardon, S.J. Archives has a superb outline of this in it's outline of the spiritual exercises).
Indifferentism is a serious sin against faith: "Indifferentism is the error of those who hold that one religion is as good as another and that all religions are equally true and pleasing to God, or that one is free to accept or reject any or all religions." Baltimore Catechism, Q. 205 Go to achurchforstarvingartists for an evangelical take on the modern fashion of indifferentism and the fundamental error implicit in all the religious leaders hanging out with each other (as in the cover image of the book above). Remember that Christ transfigured himself upon the mountain with Moses and Elijah: He shone, radiant like the sun; they vanished! Matthew 17:1-8 "'Arise, and do not be afraid.' But lifting up their eyes, they saw no one but Jesus only."
Below I quote from a miniature hand-made New Year's card which a Sister of the Missionaries of Charity gave to me yesterday at 12:50 AM (after saying midnight Mass for them). The battle language is striking: "the grace to live with a fighting spirit!"--With the Spirit of the living God, who fights, armed with innocence, involvement, transparency, gentleness and every sweetness: the veritable infant! The Prince of Peace! But very much a Prince! He stands for something! He stands for everything! He stands for everything because He stands for One Thing: God!
Happy New Year
Year of Faith
And Joy and
Peace
Let us ask Our Lady
for the grace to live
during this New Year
with a fighting Spirit,
as if it were the
last that God was
going to give us.