e alone; which
is attributed to the Holy Ghost, according to 1 Cor. 12:4: "There are
diversities of graces, but the same Spirit." Wherefore Augustine says
(Enchiridion xl): "The manner in which Christ was born of the Holy
Ghost . . . suggests to us the grace of God, whereby man, without any
merits going before, in the very beginning of his nature when he
began to exist was joined to God the Word, into so great unity of
Person, that He Himself should be the Son of God."

Thirdly, because this is befitting the term of the Incarnation. For
the term of the Incarnation was that that man, who was being
conceived, should be the Holy one and the Son of God. Now, both of
these are attributed to the Holy Ghost. For by Him men are made to be
sons of God, according to Gal. 4:6: "Because you are sons, God hath
sent the Spirit of His Son into your [Vulg.: 'our'] hearts, crying:
Abba, Father." Again, He is the "Spirit of sanctification," according
to Rom. 1:4. Therefore, just as other men are sanctified spiritually
by the Holy Ghost; so as to be the adopted sons of God, so was Christ
conceived in sanctity by the Holy Ghost, so as to be the natural Son
of God. Hence, according to a gloss on Rom. 1:4, the words, "Who was
predestinated the Son of God, in power," are explained by what
immediately follows: "According to the Spirit of sanctification, i.e.
through being conceived of the Holy Ghost." And the Angel of the
Annunciation himself, after saying, "The Holy Ghost shall come upon
thee," draws the conclusion: "Therefore also the Holy which shall be
born of thee shall be called the Son of God."

Reply Obj. 1: The work of the conception is indeed common to the
whole Trinity; yet in some way it is attributed to each of the
Persons. For to the Father is attributed authority in regard to the
Person of the Son, who by this conception took to Himself (human
nature). The taking itself (of human nature) is attributed to the
Son: but the formation of the body taken by the Son is attributed to
the Holy Ghost. For the Holy Ghost is the Spirit of the Son,
according to Gal. 4:6: "God sent the Spirit of His Son." For just as
the power of the soul which is in the semen, through the spirit
enclosed therein, fashions the body in the generation of other men,
so the Power of God, which is the Son Himself, according to 1 Cor.
1:24: "Christ, the Power of God," through the Holy Ghost formed the
body which He assumed. This is also shown by the words of the