Hebrews 7:10

"For he was still in the loins." The seed that was the beginning of Levi's life was within the loins of Abraham. Indeed, Isaac was the cause and origin of Levi.

Why is it that Scripture reports, for the sake of the tremendous difference between the priesthood of Christ and that of Levi, that Levi paid a tithe to Melchizedek when he was in the loins of Abraham, since Christ was also there, and so both Levi and Christ paid the tithe? Unless it is because we should understand that, in some other way, Christ was not there? But who would deny that Christ was there according to the flesh? Then he was not there according to the soul, for the soul of Christ did not originate through the transmission of the sin of Adam, or else he would have been there.…Levi was surely there in the loins of Abraham in accordance with the transmission of human seed by which he would enter into his mother; Christ was not there through that cause, although the flesh of Mary was. Thus, neither Levi nor Christ were present there according to the soul, but both of them were there according to the flesh. Levi was there according to fleshly desire, while Christ was there only according to his physical substance. For in a seed, there is both a visible physicality and an invisible principle. Both ran their course from Abraham, even from Adam himself, all the way to the body of Mary, since that too was conceived and born in the normal way. So Christ assumed the physical substance of flesh from the flesh of the virgin, but the reason for his conception did not come from a man's seed, but from a much different source—from above. So for this reason, the flesh which he assumed from his mother was also present in the loins of Abraham. So Levi paid a tithe in Abraham, who, although he was only there according to the flesh, was still there in the loins of Abraham, as Abraham also was once in the loins of his own father. […]

We now advance in reply to such as argue that one who is born of a baptized man ought himself to be regarded as already baptized. "For why," they ask, "could he not have been baptized in the loins of his father, when, according to the Epistle to the Hebrews, Levi was able to pay tithes in the loins of Abraham?" They who propose this argument ought to observe that Levi did not on this account subsequently not pay tithes, because he had paid tithes already in the loins of Abraham, but because he was ordained to the office of the priesthood in order to receive tithes, not to pay them; otherwise neither would his brethren, who all contributed their tithes to him, have been tithed—because they too, whilst in the loins of Abraham, had already paid tithes to Melchisedec.

How? "For he was yet in his loins when Melchisedec met him," i.e. Levi was in him, although he was not yet born. And he said not the Levites but Levi. Hast thou seen the superiority? Hast thou seen how great is the interval between Abraham and Melchisedec, who bears the type of our High Priest? And he shows that the superiority had been caused by authority, not necessity. For the one paid the tithe, which indicates the priest: the other gave the blessing, which indicates the superior. This superiority passes on also to the descendants. In a marvelous and triumphant way he cast out the Jewish system. On this account he said, "Ye are become dull," because he wished to lay these foundations, that they might not start away. Such is the wisdom of Paul, first preparing them well, he so leads them into what he wishes. For the human race is hard to persuade, and needs much attention, even more than plants. Since in that case there is only the nature of material bodies, and earth, which yields to the hands of the husbandmen: but in this there is will, which is liable to many alterations, and now prefers this, now that. For it quickly turns to evil.

Through Abraham, who gave him the tenth part, the house of Levi, which had to be generated by him, took the tenth part in him. The Levites, even though they took the tenth part, did not take it from strangers but received the tenth part …