John 1:33

33.Upon whom thou shalt see the Spirit descending. Here a difficult question arises; for if John did not know Christ, why does he refuse to admit him to baptism? To a person whom he did not know he would not say, I ought rather to be baptized by thee, (Matthew 3:14.) Some reply, that he knew him to such an extent as to regard him with the reverence due to a distinguished Prophet, but was not aware that he wasthe Son of God. But this is a poor solution of the difficulty, for every man ought to obey the calling of God without any respect of persons. No rank or excellence of man ought to prevent us from doing our duty, and therefore John would have shown disrespect to God and to his baptism, if he had spoken in this manner to any other person than the Son of God. it follows that he must have previously known Christ.

In the first place, it ought to be observed, that the knowledge here mentioned is that which arises from personal and long acquaintance. Although he recognizes Christ whenever he sees him, still it does not cease to be true that they were not known to each other according to the ordinary custom of men, for the commencement of his knowledge proceeded from God. But the question is not yet fully answered; for he says that the sight of the Holy Spirit was the mark by which he was pointed out to him. Now he had not yet seen the Spirit, when he had addressed Christ as the Son of God. For my own part, I willingly embrace the opinion of those who think that this sign was added for confirmation, and that it was not so much for the sake of John as for the sake of us all. John indeed saw it, but it was rather for others than for himself. Bucer appropriately quotes that saying of Moses,

This shall be a sign to you, that after three days journey, you shall sacrifice to me on the mountain, (Exodus 3:12.)

Undoubtedly, when they were going out, they already knew that God would conduct and watch over their deliverance; but this was a confirmation a posteriori, as the phrase is; that is, from the event, after it had taken place. In like manner, this came as an addition to the former revelation which had been given to John.