1: What is accidental to a thing does not diversify its
essence. Now bodily washing with water is essential to Baptism:
wherefore Baptism is called a "laver," according to Eph. 5:26:
"Cleansing it by the laver of water in the word of life." But that
the washing be done this or that way, is accidental to Baptism. And
consequently such diversity does not destroy the oneness of Baptism.

Reply Obj. 2: Christ's burial is more clearly represented by
immersion: wherefore this manner of baptizing is more frequently in
use and more commendable. Yet in the other ways of baptizing it is
represented after a fashion, albeit not so clearly; for no matter how
the washing is done, the body of a man, or some part thereof, is put
under water, just as Christ's body was put under the earth.

Reply Obj. 3: The principal part of the body, especially in relation
to the exterior members, is the head, wherein all the senses, both
interior and exterior, flourish. And therefore, if the whole body
cannot be covered with water, because of the scarcity of water, or
because of some other reason, it is necessary to pour water over the
head, in which the principle of animal life is made manifest.

And although original sin is transmitted through the members that
serve for procreation, yet those members are not to be sprinkled in
preference to the head, because by Baptism the transmission of
original sin to the offspring by the act of procreation is not
deleted, but the soul is freed from the stain and debt of sin which
it has contracted. Consequently that part of the body should be
washed in preference, in which the works of the soul are made
manifest.

Nevertheless in the Old Law the remedy against original sin was
affixed to the member of procreation; because He through Whom
original sin was to be removed, was yet to be born of the seed of
Abraham, whose faith was signified by circumcision according to Rom.
4:11.
_______________________

EIGHTH ARTICLE [III, Q. 66, Art. 8]

Whether Trine Immersion Is Essential to Baptism?

Objection 1: It seems that trine immersion is essential to Baptism.
For Augustine says in a sermon on the Symbol, addressed to the
Neophytes: "Rightly were you dipped three times, since you were
baptized in the name of the Trinity. Rightly were you dipped three
times, because you were baptized in the name of Jesus Christ, Who on
the third day rose again from the dead. For that thrice repeated
immersion reproduces the bu