5 “From that time forth began He to show unto them that He must suffer. From that time.” What time? When He had fixed the doctrine in them; when He had brought in the beginning of the Gentiles.
But not even so did they understand what He said. “For the saying,” it is said, “was hid from them;” and they were as in a kind of perplexity, not knowing that He must rise again. Therefore He rather dwells on the difficulties, and enlarges His discourse, that He may open their mind, and they may understand what it can be that He speaks of.
“But they understood not, but the saying was hid from them, and they feared to ask this;” not whether He should die, but how, and in what manner, and what this mystery could be. For they did not even know what was this same rising again, and supposed it much better not to die. Therefore, the rest being troubled and in perplexity, Peter again, in his ardor, alone ventures to discourse of these things; and not even he openly, but when he had taken Him apart; that is, having separated himself from the rest of the disciples; and he says, “Be it far from You, Lord, this shall not be unto You.” What ever is this? He that obtained a revelation, he that was blessed, has he so soon fallen away, and suffered overthrow, so as to fear His passion? And what marvel, that one who had not on these points received any revelation, should have that feeling? Yea, to inform you that not of himself did he speak those other things either, see in these matters that were not revealed to him how he is confounded and overthrown, and being told ten thousand times, knows not what the saying can mean.
For that He is Son of God he had learned, but what the mystery of the cross and of the resurrection might be, was not yet manifest to him: for “the saying,” it is said, “was hid from them.”
Do you see that with just cause He bade them not declare it to the rest? For if it so confounded them, who must needs be made aware of it, what would not all others have felt?
Source: Homilies on the Gospel of St. Matthew (New Advent)