Now, since we are upon the subject of Peter, and of the teachers of Christianity to the circumcision, I do not deem it out of place to quote a certain declaration of Jesus taken from the Gospel according to John, and to give the explanation of the same. For it is there related that Jesus said: “I have yet many things to say unto you, but you cannot bear them now. Howbeit when He, the Spirit of truth, has come, He will guide you into all the truth: for He shall not speak of Himself; but whatsoever He shall hear, that shall He speak.” And when we inquire what were the “many things” referred to in the passage which Jesus had to say to His disciples, but which they were not then able to bear, I have to observe that, probably because the apostles were Jews, and had been trained up according to the letter of the Mosaic law, He was unable to tell them what was the true law, and how the Jewish worship consisted in the pattern and shadow of certain heavenly things, and how future blessings were foreshadowed by the injunctions regarding meats and drinks, and festivals, and new moons, and sabbaths.
These were many of the subjects which He had to explain to them; but as He saw that it was a work of exceeding difficulty to root out of the mind opinions that have been almost born with a man, and amid which he has been brought up till he reached the period of maturity, and which have produced in those who have adopted them the belief that they are divine, and that it is an act of impiety to overthrow them; and to demonstrate by the superiority of Christian doctrine, that is, by the truth, in a manner to convince the hearers, that such opinions were but “loss and dung,” He postponed such a task to a future season— to that, namely, which followed His passion and resurrection.
For the bringing of aid unseasonably to those who were not yet capable of receiving it, might have overturned the idea which they had already formed of Jesus, as the Christ, and the Son of the living God. And see if there is not some well-grounded reason for such a statement as this, “I have many things to say unto you, but you cannot bear them now;” seeing there are many points in the law which require to be explained and cleared up in a spiritual sense, and these the disciples were in a manner unable to bear, having been born and brought up among Jews.
I am of opinion, moreover, that since these rites were typical, and the truth was that which was to be taught them by the Holy Spirit, these words were added, “When He has come who is the Spirit of truth, He will lead you into all the truth;” as if He had said, into all the truth about those things which, being to you but types, you believed to constitute a true worship which you rendered unto God. And so, according to the promise of Jesus, the Spirit of truth came to Peter, saying to him, with regard to the four-footed beasts, and creeping things of the earth, and fowls of the air: “Arise, Peter; kill, and eat.”
And the Spirit came to him while he was still in a state of superstitious ignorance; for he said, in answer to the divine command, “Not so Lord; for I have never yet eaten anything common or unclean.” He instructed him, however, in the true and spiritual meaning of meats, by saying, “What God has cleansed, call not common.” And so, after that vision, the Spirit of truth, which conducted Peter into all the truth, told him the many things which he was unable to bear when Jesus was still with him in the flesh. But I shall have another opportunity of explaining those matters, which are connected with the literal acceptation of the Mosaic law.
Source: Against Celsus (New Advent)