Himself, Who is
sent. For He is the Word of God: and the word is conceived without
any interior corruption: indeed, interior corruption is incompatible
with perfect conception of the word. Since therefore flesh was so
assumed by the Word of God, as to be the flesh of the Word of God, it
was fitting that it also should be conceived without corruption of
the mother.

Thirdly, this was befitting to the dignity of Christ's humanity in
which there could be no sin, since by it the sin of the world was
taken away, according to John 1:29: "Behold the Lamb of God" (i.e.
the Lamb without stain) "who taketh away the sin of the world." Now
it was not possible in a nature already corrupt, for flesh to be born
from sexual intercourse without incurring the infection of original
sin. Whence Augustine says (De Nup. et Concup. i): "In that union,"
viz. the marriage of Mary and Joseph, "the nuptial intercourse alone
was lacking: because in sinful flesh this could not be without
fleshly concupiscence which arises from sin, and without which He
wished to be conceived, Who was to be without sin."

Fourthly, on account of the very end of the Incarnation of Christ,
which was that men might be born again as sons of God, "not of the
will of the flesh, nor of the will of man, but of God" (John 1:13),
i.e. of the power of God, of which fact the very conception of Christ
was to appear as an exemplar. Whence Augustine says (De Sanct.
Virg.): "It behooved that our Head, by a notable miracle, should be
born, after the flesh, of a virgin, that He might thereby signify
that His members would be born, after the Spirit, of a virgin Church."

Reply Obj. 1: As Bede says on Luke 1:33: Joseph is called the father
of the Saviour, not that he really was His father, as the Photinians
pretended: but that he was considered by men to be so, for the
safeguarding of Mary's good name. Wherefore Luke adds (Luke 3:23):
"Being, as it was supposed, the son of Joseph."

Or, according to Augustine (De Cons. Evang. ii), Joseph is called the
father of Christ just as "he is called the husband of Mary, without
fleshly mingling, by the mere bond of marriage: being thereby united
to Him much more closely than if he were adopted from another family.
Consequently that Christ was not begotten of Joseph by fleshly union
is no reason why Joseph should not be called His father; since he
would be the father even of an adopted son not born of his wife."

Reply Obj. 2: As Jerome say