angel:
"The Holy Ghost shall come upon thee," as it were, in order to
prepare and fashion the matter of Christ's body; "and the Power of
the Most High," i.e. Christ, "shall overshadow thee--that is to say,
the incorporeal Light of the Godhead shall in thee take the corporeal
substance of human nature: for a shadow is formed by light and body,"
as Gregory says (Moral. xviii). The "Most High" is the Father, whose
Power is the Son.

Reply Obj. 2: The mission refers to the Person assuming, who is sent
by the Father; but the conception refers to the body assumed, which
is formed by the operation of the Holy Ghost. And therefore, though
mission and conception are in the same subject; since they differ in
our consideration of them, mission is attributed to the Father, but
the accomplishment of the conception to the Holy Ghost; whereas the
assumption of flesh is attributed to the Son.

Reply Obj. 3: As Augustine says (QQ. Vet. et Nov. Test., qu. 52):
"This may be understood in two ways. For, first, Christ's house is
the Church, which He built with His blood. Secondly, His body may be
called His house, just as it is called His temple . . . and what is
done by the Holy Ghost is done by the Son of God, because Theirs is
one Nature and one Will."
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SECOND ARTICLE [III, Q. 32, Art. 2]

Whether It Should Be Said That Christ Was Conceived of (_de_) the
Holy Ghost?

Objection 1: It would seem that we should not say that Christ was
conceived of (_de_) the Holy Ghost. Because on Rom. 11:36: "For of
Him (_ex ipso_) and by Him, and in Him, are all things," the gloss of
Augustine says: "Notice that he does not say, 'of Him' (_de ipso_),
but 'of Him' (_ex ipso_). For of Him (_ex ipso_), are heaven and
earth, since He made them: but not of Him [de ipso, since they are
not made of His substance." But the Holy Ghost did not form Christ's
body of (_de_) His own substance. Therefore we should not say that
Christ was conceived of (_de_) the Holy Ghost.

Obj. 2: Further, the active principle of (_de_) which something is
conceived is as the seed in generation. But the Holy Ghost did not
take the place of seed in Christ's conception. For Jerome says
(Expos. Cathol. Fidei) [*Written by Pelagius]: "We do not say, as
some wicked wretches hold, that the Holy Ghost took the place of
seed: but we say that Christ's body was wrought," i.e. formed, "by
the power and might of the Creator." Therefore we should not say that
Chris