Hebrews 8:3

"For every high priest." For he says, it is proper for the high priests to offer gifts and sacrifices (for this is why they are established), it is necessary for the Christ also to offer something, I mean sacrifices. Therefore, he took on his own …

They do not understand, that not even the proudest of spirits themselves could rejoice in the honor of sacrifices, unless a true sacrifice was due to the one true God, in whose stead they desire to be worshipped: and that this cannot be rightly offered except by a holy and righteous priest; nor unless that which is offered be received from those for whom it is offered; and unless also it be without fault, so that it may be offered for cleansing the faulty. This at least all desire who wish sacrifice to be offered for themselves to God. Who then is so righteous and holy a priest as the only Son of God, who had no need to purge His own sins by sacrifice, neither original sins, nor those which are added by human life? And what could be so fitly chosen by men to be offered for them as human flesh? And what so fit for this immolation as mortal flesh? And what so clean for cleansing the faults of mortal men as the flesh born in and from the womb of a virgin, without any infection of carnal concupiscence? And what could be so acceptably offered and taken, as the flesh of our sacrifice, made the body of our priest? In such wise that, whereas four things are to be considered in every sacrifice,—to whom it is offered, by whom it is offered, what is offered, for whom it is offered,—the same One and true Mediator Himself, reconciling us to God by the sacrifice of peace, might remain one with Him to whom He offered, might make those one in Himself for whom He offered, Himself might be in one both the offerer and the offering.

It is proper for a high priest to offer gifts to the God of all. For this reason, the only begotten, when he was made man and assumed our nature, offered it for us.

In the next place that thou mayest understand that he used the word "minister" of the manhood, observe how he again indicates it: "For" (he says) "every high priest is ordained to offer both gifts and sacrifices, wherefore it is of necessity that this man have somewhat also to offer." Do not now, because thou hearest that He sitteth, suppose that His being called High Priest is mere idle talk. For the former, viz. His sitting, belongs to the dignity of the Godhead, but this to His great lovingkindness, and His tender care for us. On this account he repeatedly urges this very thing, and dwells more upon it: for he feared lest the other truth should overthrow it. That is, lest the belief of His Godhead should undermine our belief in His true manhood. Therefore he again brings down his discourse to this: since some were enquiring why He died. He was a Priest. But there is no Priest without a sacrifice. It is necessary then that He also should have a sacrifice.

I am very much surprised how they have ventured to entertain the idea that the Word became man in consequence of his nature. For, if this were so, the commemoration of Mary would be superfluous. For nature has no conception of a virgin bearing apart from a man.