7 Nor ought it, my dearest brother, to disturb any one who is faithful and mindful of the Gospel, and retains the commands of the apostle who forewarns us; if in the last days certain persons, proud, contumacious, and enemies of God's priests, either depart from the Church or act against the Church, since both the Lord and His apostles have previously foretold that there should be such. Nor let any one wonder that the servant placed over them should be forsaken by some, when His own disciples forsook the Lord Himself, who performed such great and wonderful works, and illustrated the attributes of God the Father by the testimony of His doings.
And yet He did not rebuke them when they went away, nor even severely threaten them; but rather, turning to His apostles, He said, “Will you also go away?” manifestly observing the law whereby a man left to his own liberty, and established in his own choice, himself desires for himself either death or salvation. Nevertheless, Peter, upon whom by the same Lord the Church had been built, speaking one for all, and answering with the voice of the Church, says, “Lord, to whom shall we go?
You have the words of eternal life; and we believe, and are sure that You are the Christ, the Son of the living God:” signifying, doubtless, and showing that those who departed from Christ perished by their own fault, yet that the Church which believes on Christ, and holds that which it has once learned, never departs from Him at all, and that those are the Church who remain in the house of God; but that, on the other hand, they are not the plantation planted by God the Father, whom we see not to be established with the stability of wheat, but blown about like chaff by the breath of the enemy scattering them, of whom John also in his epistle says, “They went out from us, but they were not of us; for if they had been of us, no doubt they would have continued with us.” Paul also warns us, when evil men perish out of the Church, not to be disturbed, nor to let our faith be lessened by the departure of the faithless.
“For what,” he says, “if some of them have departed from the faith? Hath their unbelief made the faith of God of none effect? God forbid! For God is true, but every man a liar.”
Source: The Epistles of Cyprian (New Advent)