8 “From that time many of His disciples went back, and walked no more with Him.” Went back, but after Satan, not after Christ. For our Lord Christ once addressed Peter as Satan, rather because he wished to precede his Lord, and to give counsel that He should not die, He who had come to die, that we might not die for ever; and He says to him, “Get behind me, Satan; for you savor not the things that be of God, but the things that be of men.” He did not drive him back to go after Satan, and so called him Satan; but He made him go behind Himself, that by walking after his Lord he should not be a Satan.
But these went back in the same manner as the apostle says of certain women: “For some are turned back after Satan.” They walked not further with Him. Behold, cut off from the body, for perhaps they were not in the body, they have lost life. They must be reckoned among the unbelieving, notwithstanding they were called disciples. Not a few, but “many went back.” This happened, it may be, for our consolation. For sometimes it happens that a man may declare the truth, and that what he says may not be understood, and so they that hear it are offended and go away.
Now the man regrets that he had spoken that truth, and he says to himself, “I ought not to have spoken so, I ought not to have said this.” Behold; it happened to the Lord: He spoke, and lost many; He remained with few. But yet He was not troubled, because He knew from the beginning who they were that believed and that believed not. If it happen to us, we are sorely perplexed. Let us find comfort in the Lord, and yet let us speak words with prudence.
Source: Tractates on the Gospel of John (New Advent)