Later on, however, there arose the error of certain schismatics and heretics who rebaptized: as Augustine (Super. Joan., cf. De Haeres. lxix) relates of the Donatists. Wherefore, in detestation of their error, only one immersion was ordered to be made, by the (fourth) council of Toledo, in the acts of which we read: "In order to avoid the scandal of schism or the practice of heretical teaching let us hold to the single baptismal immersion."
But now that this motive has ceased, trine immersion is universally observed in Baptism: and consequently anyone baptizing otherwise would sin gravely, through not following the ritual of the Church. It would, however, be valid Baptism.
Reply Obj. 1: The Trinity acts as principal agent in Baptism. Now the likeness of the agent enters into the effect, in regard to the form and not in regard to the matter. Wherefore the Trinity is signified in Baptism by the words of the form. Nor is it essential for the Trinity to be signified by the manner in which the matter is used; although this is done to make the signification clearer.
In like manner Christ's death is sufficiently represented in the one immersion. And the three days of His burial were not necessary for our salvation, because even if He had been buried or dead for one day, this would have been enough to consummate our redemption: yet those three days were ordained unto the manifestation of the reality of His death, as stated above (Q. 53, A. 2). It is therefore clear that neither on the part of the Trinity, nor on the part of Christ's Passion, is the trine immersion essential to the sacrament.
Reply Obj. 2: Pope Pelagius understood the trine immersion to be ordained by Christ in its equivalent; in the sense that Christ commanded Baptism to be conferred "in the name of the Father, and of the Son, and of the Holy Ghost." Nor can we argue from the form to the use of the matter, as stated above (ad 1).
Reply Obj. 3: As stated above (Q. 64, A. 8), the intention is essential to Baptism. Consequently, one Baptism results from the intention of the Church's minister, who intends to confer one Baptism by a trine immersion. Wherefore Jerome says on Eph. 4:5, 6: "Though the Baptism," i.e. the immersion, "be thrice repeated, on account of the mystery of the Trinity, yet it is reputed as one Baptism."
If, however, the intention were to confer one Baptism at each immersion together with the repetition of the words of the form, it would be a sin, in itself, because it would be a repetition of Baptism. _______________________
NINTH ARTICLE [III, Q. 66, Art. 9]
Whether Baptism May Be Reiterated?
Objection 1: It seems that Baptism may be reiterated. For Baptism was instituted, seemingly, in order to wash away sins. But sins are reiterated. Therefore much more should Baptism be reiterated: because Christ's mercy surpasses man's guilt.