Mary descended from David

And that the virgin of whom it behoved Christ to be born (as we have above mentioned) must derive her lineage of the seed of David, the prophet in subsequent passages evidently asserts. “And there shall be born,” he says, “a rod from the root of Jesse”—which rod is Mary—“and a flower shall ascend from his root: and there shall rest upon him the Spirit of God, the spirit of wisdom and understanding, the spirit of discernment and piety, the spirit of counsel and truth; the spirit of God’s fear shall fill Him.”

Tertullian
Against the Jews
Chapter IX

Lewis on the fact that the ancients understood the natural world as not being miraculous

“Joseph didn’t want to leave Mary because he didn’t know where babies came from but because he knew precisely where they did.”

Kallistos Ware on Lewis

Lewis’s actual quote

“He was the husband of the Virgin Mary. If you’ll read the story in the Bible you’ll find that when he saw his fiancée was going to have a baby he decided to cry off the marriage. Why did he do that?”

“Wouldn’t most men?”

“Any man would’, said I, ‘provided he knew the laws of Nature — in other words, provided he knew that a girl doesn’t ordinarily have a baby unless she’s been sleeping with a man. But according to your theory people in the old days didn’t know that Nature was governed by fixed laws. I’m pointing out that the story shows that St Joseph knew that law just as well as you do.”

“But he came to believe in the Virgin Birth afterwards, didn’t he?”

“Quite. But he didn’t do so because he was under any illusion as to where babies came from in the ordinary course of Nature. He believed in the Virgin Birth as something supernatural. He knew Nature works in fixed, regular ways: but he also believed that there existed something beyond Nature which could interfere with her workings — from outside, so to speak.”

C.S. Lewis
Religion and Science, God in the Dock

Belief in the Virgin Birth is necessary for salvation

Whosoever wishes to see Jesus the Son of David must believe in Him; through the Virgin’s birth. He who will not believe this will not hear from Him the salutation, “Thy faith hath saved thee.” And so he will remain blind, falling into Antithesis after Antithesis, which mutually destroy each other, just as “the blind man leads the blind down into the ditch.”

Tertullian
The Five Books Against Marcion, Book IV
Chapter XXXVI

The Magi worshipped Christ as God

For the East generally regarded the magi as kings; and Damascus was anciently deemed to belong to Arabia, before it was transferred to Syrophœnicia on the division of the Syrias (by Rome). Its riches Christ then received, when He received the tokens thereof in the gold and spices; while the spoils of Samaria were the magi themselves. These having discovered Him and honored Him with their gifts, and on bended knee adored Him as their God and King, through the witness of the star which led their way and guided them, became the spoils of Samaria, that is to say, of idolatry, because, as it is easy enough to see, they believed in Christ.

Tertullian
The Five Books Against Marcion, Book III
Chapter XIII

God was born

Come then, wind up your cavils against the most sacred and reverend works of nature; inveigh against all that you are; destroy the origin of flesh and life; call the womb a sewer of the illustrious animal—in other words, the manufactory for the production of man; dilate on the impure and shameful tortures of parturition, and then on the filthy, troublesome, contemptible issues of the puerperal labour itself! But yet, after you have pulled all these things down to infamy, that you may affirm them to be unworthy of God, birth will not be worse for Him than death, infancy than the cross, punishment than nature, condemnation than the flesh. If Christ truly suffered all this, to be born was a less thing for Him. If Christ suffered evasively, as a phantom; evasively, too, might He have been born. Such are Marcion’s chief arguments by which he makes out another Christ; and I think that we show plainly enough that they are utterly irrelevant, when we teach how much more truly consistent with God is the reality rather than the falsehood of that condition in which He manifested His Christ. Since He was “the truth,” He was flesh; since He was flesh, He was born.

Tertullian
The Five Books Against Marcion, Book III
Chapter XI

The flesh became a vestment for God himself

But yet, if we look steadily into the subject, there is really no substance which is worthy of becoming a vestment for God. Whatsoever He is pleased to clothe Himself withal, He makes worthy of Himself—only without untruth. Therefore how comes it to pass that he should have thought the verity of the flesh, rather than its unreality, a disgrace?

Tertullian
The Five Books Against Marcion, Book III
Chapter X

Christ regenerated our lives through incarnation and resurrection

…none but Christ could become incarnate by being born of the flesh in order that by His own nativity He might regenerate our birth, and might further by His death also dissolve our death, by rising again in that flesh in which, that He might even die, He was born.

Tertullian
The Five Books Against Marcion, Book III
Chapter IX

God became man so man might learn to act as god

God held converse with man, that man might learn to act as God. God dealt on equal terms with man, that man might be able to deal on equal terms with God. God was found little, that man might become very great. You who disdain such a God, I hardly know whether you ex fide believe that God was crucified.

Tertullian
The Five Books Against Marcion, Book II
Chapter XXVIII

Christ, as a human being, is our God

Now we believe that Christ did ever act in the name of God the Father; that He actually from the beginning held intercourse with (men); actually communed with patriarchs and prophets; was the Son of the Creator; was His Word; whom God made His Son by emitting Him from His own self, and thenceforth set Him over every dispensation and (administration of) His will, making Him a little lower than the angels, as is written in David.

…For even in the Old Testament He had declared, “No man shall see me, and live.” He means that the Father is invisible, in whose authority and in whose name was He God who appeared as the Son of God. But with us Christ is received in the person of Christ, because even in this manner is He our God.

Tertullian
The Five Books Against Marcion, Book II
Chapter XXVII