Psalms 110:4

Then immediately again with reference to his humanity he says: The Lord hath sworn and will not repent, Thou art a priest for ever after the order of Melchisedec; for deity does not exercise the priestly office or render worship, but is rather itself worshipped and the recipient of sacred services.

The Lord has sworn, and he will not repent. "The Lord has sworn." In what he says, "he has sworn," it is not to the one who was begotten before the morning star, but he swore to the one who after the morning star was born of the Virgin. You are a priest forever according to the order of Melchizedech. It is superfluous for us to wish to interpret this little verse, since the holy Apostle argued about it most fully to the Hebrews. For this Melchizedech himself is without father, without mother, without lineage. And there he interprets most diligently why "without father, without mother, without lineage"; and it has been said by all churchmen that he is called "without father" according to the flesh, and "without mother" according to God. Let us therefore interpret only this: You are a priest forever according to the order of Melchizedech. Let us say only this, why he said "according to the order." "According to the order": by no means will you be a priest according to the Jewish victims, but you will be a priest according to the order of Melchizedech. For just as Melchizedech, king of Salem, offered bread and wine, so you too will offer your body and blood, the true bread and the true wine. This Melchizedech gave to us these mysteries which we have. He it is who said, "Whoever shall eat my flesh and drink my blood": according to the order of Melchizedech he handed down to us his sacrament.

Daniel told a story of another hermit who used to live in the lower parts of Egypt, and who said in his simplicity that Melchizedek was the Son of God. Now when the blessed man Theophilus, the archbishop of Alexandria, heard of it, he sent a message asking the monks to bring the hermit to him. When he saw him, he realized that he was a man of vision and that every thing that he had asked for God had given him, and that he had only said this out of simplicity. The archbishop dealt with him wisely in the following manner, saying, 'Abba, pray to God for me, because I have begun to think that Melchizedek was the Son of God,' and he added, 'It cannot be true, for the high priest of God was a man. But because I had doubts in my mind about this, I sent for you to pray to God for me that He may reveal the truth of the matter to you.' Then, because the hermit had confidence in the power of prayer, he said to him firmly, 'Wait three days, and I will ask God about this and then I shall be able to tell you who Melchizedek was.' So the hermit went away, and returned after three days, and said to the blessed Archbishop Theophilus, 'Melchizedek was a man.' The Archbishop said unto him, 'How do you know that, abba?' The hermit said, 'God showed me all the Patriarchs, one by one, and they passed before me one after the other, from Adam to Melchizedek, and an angel said to me, "This is Melchizedek." That is indeed how the truth of this matter appeared tome.' The hermit went away, and he himself proclaimed that Melchizedek was a man, and the blessed Theophilus rejoiced greatly.