Hebrews 7:9

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 from themselves; in fact, they took the tenth part from their brothers, the sons of Abraham. Therefore, Abraham, to whom the promise of priesthood was made, gave the tenth part to Melchizedek, who was not inscribed in the Levitic generation. And to Abraham it had been promised that all nations would have been blessed in him. So why did he need the blessing of an uncircumcised man? Does not this show and prove that, if Abraham had not been inferior to Melchizedek, he would not have demanded to be blessed by him? And so the mortal sons receive the tenth part, and in the same manner Melchizedek, who was mortal, lived at that time to be a witness for Abraham, for the indisputably true Melchizedek’s blessing destined to the seed of Abraham.

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.

We now advance in reply to those who 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 it was not because he had paid tithes already in the loins of Abraham that Levi did not subsequently pay tithes, but because he was ordained to the office of the priesthood in order to receive tithes, not pay them. Otherwise, neither would his brethren, who all contributed their tithes to him, have been tithed—because they too, while in the loins of Abraham, had already paid tithes to Melchizedek.

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. In other words, he was born of his father Abraham in the same way that Abraham was born of his own father, namely through the law at work in his members fighting against the "law of his mind" and an invisible concupiscence, though the chaste and noble rights of marriage do not permit it to grow strong except insofar as these things are able to make provision for the continuation of the human race.
But he who acquired his flesh not as a rotting wound, but as the source of healing, did not himself also pay a tithe in that way. Since the paying of the tithe served to prefigure the source of healing, the one who would be cured paid the tithe in the flesh of Abraham, but not the one from whom healing would come. For the same flesh, not only that of Abraham, but also that of the first man taken from the earth, contained in itself at the same time both the wound caused by transgression and the medicine for that wound. The wound of sin was at work in the law of the fleshly members fighting against the law of the mind; this law was being transcribed upon all flesh begotten by the principle of a human seed. But the medicine for the wound was also in that flesh, which was assumed without any deed of concupiscence, assumed only in the physical material of the flesh from the Virgin through a divine principle of conception and formation for the sake of a participation in our death not due to his own iniquity and as an example of resurrection that is not deceptive.…
The soul of Christ is from the original soul only if it has not contracted the stain of sin; but if it could not be from that source without the guilt of sin, it has not come from that soul.

He says, that because Abraham paid tithes, also Levi “who receives tithes himself was made to tithe,” that is, he gave a tithe. We must underscore the phrase “through Abraham,” so that the meaning does not suffer violence. For because Abraham was made to tithe, in a certain sense also Levi, being still “in his loins” has been made to tithe.

He says, “even though they have come out of the loins of Abraham.” Although the Levites are in all other respects equal in rank with the other tribes, nonetheless because the other tribes give tithes while the Levites receive them, the Levites are clearly superior. But if this is the case, then clearly also the same principle applies to Abraham and Melchizedek, the giver and recipient, respectively. Consequently, the type of Christ [Melchizedek] is greater than the patriarch Abraham. But if he is greater than Abraham, he is much greater than the priests. And if the type is greater than Abraham, what would anyone say concerning Christ himself?

Therefore, it is said that Abraham gave tithes, even Levi who receives tithes was himself tithed, that is, he gave tithes. Thus, concerning those words, "through Abraham," a distinction must be made so that the meaning becomes completely undeniable. For because of Abraham's tithing, in a certain way, Levi, who was still in his loins, was also tithing.

It shows that there was great excellence in Melchizedek, and therefore he received tithes, and it says: Even the Levites receive; but this is not great, for they die; who, then, envies the honor of one who is about to die? However, Melchizedek, even if he received testimony that he lives, received tithes. And what it says about him receiving testimony that he lives, take according to the previously given understanding, namely because Scripture did not say the end of him, but only life, it seemed to testify only that he lived; as if to say: Abraham foresaw that the death of Melchizedek would not be revealed, nor was he moved to envy him the honor of tithes, for he knew his dignity.For he has superiority. The simpler way to accept what is said is that the priesthood of the Levites dies. For it has ceased. The truths have been revealed; but the one of Melchizedek lives.
"that he lives." How then was he testified? In saying to God, "You are a priest forever according to the order of Melchizedek." (Heb. 7:17) And therefore, the distinction of Melchizedek from the priests among the Jews.
"the men who die." For he says, it is not to be assumed that at any time he became greater, but after this he was established in a lesser portion. For in the law, the Levites receive tithes, and those who die pass on their value to others. So that the same ones are not always those who receive the tithe. But here, we have learned nothing of the sort. Therefore, he should always be regarded as superior, if indeed he remains always upon his own value. For he remains, as I said, an everlasting priest, having taken this from Scripture, as in the saying, "You are a priest forever according to the order of Melchizedek," (Heb. 7:17) showing that he is such an everlasting priest.
"and so to speak." So that they may not say, "And what," he says, "was also tithing through Abraham?" Yes, he says. For Levi was in Abraham when he gave the tithes, even though he was not yet born. If therefore Levi was in the loins of Abraham when he gave the tithes, it is clear that Levi himself, the ancestor of the priests, gave tithes, when Abraham gave tithes. And the phrase, so to speak, means this, that, in short, or instead of saying, let me put it this way. For it seemed a bold statement to say that Levi, not yet born, was tithed by Melchizedek, and this was corrected.

"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.