8 Accordingly, when the Lord says, “I have yet many things to say unto you, but you cannot bear them now,” He means that what they were still ignorant of had afterwards to be supplied to them, and not that what they had already learned was to be subverted. And He, indeed, as I have already shown in a former discourse, could so speak, because the very things which He had taught them, had He wished to unfold them to them in the same way as they are conceived in regard to Him by the angels, their still remaining human weakness would be unable to bear.
But any spiritual man may teach another man what he knows, provided the Holy Spirit grant him an enlarged capacity for profiting, wherein also the teacher himself may get some further increase, in order that both may be taught of God. Although even among the spiritual themselves there are some, doubtless, who are of greater capacity and in a better condition than others; so that one of them attained even to things of which it is not lawful for a man to speak. Taking advantage of which, there have been some vain individuals, who, with a presumption that betrays the grossest folly, have forged a Revelation of Paul, crammed with all manner of fables, which has been rejected by the orthodox Church; affirming it to be that whereof he had said that he was caught up into the third heavens, and there heard unspeakable words “which it is not lawful for a man to utter.” Nevertheless, the audacity of such might be tolerable, had he said that he heard words which it is not as yet lawful for a man to utter; but when he said, “which it is not lawful for a man to utter,” who are they that dare to utter them with such impudence and non-success? But with these words I shall now bring this discourse to a close; whereby I would have you to be wise indeed in that which is good, but untainted by that which is evil.
Source: Tractates on the Gospel of John (New Advent)