e Father and the
Holy Ghost, otherwise the power of the three Persons would not be
one. But the Son was able to become incarnate. Therefore the Father
and the Holy Ghost were able to become incarnate.

_I answer that,_ As was said above (AA. 1, 2, 4), assumption implies
two things, viz. the act of the one assuming and the term of the
assumption. Now the principle of the act is the Divine power, and the
term is a Person. But the Divine power is indifferently and commonly
in all the Persons. Moreover, the nature of Personality is common to
all the Persons, although the personal properties are different. Now
whenever a power regards several things indifferently, it can
terminate its action in any of them indifferently, as is plain in
rational powers, which regard opposites, and can do either of them.
Therefore the Divine power could have united human nature to the
Person of the Father or of the Holy Ghost, as It united it to the
Person of the Son. And hence we must say that the Father or the Holy
Ghost could have assumed flesh even as the Son.

Reply Obj. 1: The temporal sonship, whereby Christ is said to be the
Son of Man, does not constitute His Person, as does the eternal
Sonship; but is something following upon the temporal nativity.
Hence, if the name of son were transferred to the Father or the Holy
Ghost in this manner, there would be no confusion of the Divine
Persons.

Reply Obj. 2: Adoptive sonship is a certain participation of natural
sonship; but it takes place in us, by appropriation, by the Father,
Who is the principle of natural sonship, and by the gift of the Holy
Ghost, Who is the love of the Father and Son, according to Gal. 4:6:
"God hath sent the Spirit of His Son into your hearts crying, Abba,
Father." And therefore, even as by the Incarnation of the Son we
receive adoptive sonship in the likeness of His natural sonship, so
likewise, had the Father become incarnate, we should have received
adoptive sonship from Him, as from the principle of the natural
sonship, and from the Holy Ghost as from the common bond of Father
and Son.

Reply Obj. 3: It belongs to the Father to be innascible as to eternal
birth, and the temporal birth would not destroy this. But the Son of
God is said to be sent in regard to the Incarnation, inasmuch as He
is from another, without which the Incarnation would not suffice for
the nature of mission.
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SIXTH ARTICLE [III, Q. 3, Art. 6]

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