2 Well, then, let us grant that it is so, that many can now bear those things when the Holy Spirit has been sent, which could not then, prior to His coming, be borne by the disciples: do we on that account know what it is that He would not say, as we should know it were we reading or hearing it as uttered by Himself? For it is one thing to know whether we or you could bear it; but quite another to know what it is, whether able to be borne or not. But when He Himself was silent about such things, which of us could say, It is this or that?
Or if he venture to say it, how will he prove it? For who could manifest such vanity or recklessness as when saying what he pleased to whom he pleased, even though true, to affirm without any divine authority that it was the very thing which the Lord on that occasion refused to utter? Which of us could do such a thing without incurring the severest charge of rashness—a thing which gets no countenance from prophetic or apostolic authority? For surely if we had read any such thing in the books confirmed by canonical authority, which were written after our Lord's ascension, it would not have been enough to have read such a statement, had we not also read in the same place that this was actually one of those things which the Lord was then unwilling to tell His disciples, because they were unable to bear them.
As if, for example, I were to say that the words which we read at the opening of this Gospel, “In the beginning was the Word, and the Word was with God, and the Word was God; the same was in the beginning with God:” and those which follow, because they were written afterwards, and yet without any mention of their being uttered by the Lord Jesus when He was here in the flesh, but were written by one of His apostles, to whom they were revealed by His Spirit, were some of those which the Lord would not then utter, because the disciples were unable to bear them; who would listen to me in making so rash a statement? But if in the same passage where we read the one we were also to read the other, who would not give due credence to such an apostle?
Source: Tractates on the Gospel of John (New Advent)