I answer that As stated above (Article [7], ad 1), washing with water is of itself required for Baptism, being essential to the sacrament: whereas the mode of washing is accidental to the sacrament. Consequently, as Gregory in the words above quoted explains, both single and trine immersion are lawful considered in themselves; since one immersion signifies the oneness of Christ's death and of the Godhead; while trine immersion signifies the three days of Christ's burial, and also the Trinity of Persons.
But for various reasons, according as the Church has ordained, one mode has been in practice, at one time, the other at another time. For since from the very earliest days of the Church some have had false notions concerning the Trinity, holding that Christ is a mere man, and that He is not called the "Son of God" or "God" except by reason of His merit, which was chiefly in His death; for this reason they did not baptize in the name of the Trinity, but in memory of Christ's death, and with one immersion. And this was condemned in the early Church. Wherefore in the Apostolic Canons (xlix) we read: "If any priest or bishop confer baptism not with the trine immersion in the one administration, but with one immersion, which baptism is said to be conferred by some in the death of the Lord, let him be deposed": for our Lord did not say, "Baptize ye in My death," but "In the name of the Father and of the Son, and of the Holy Ghost."
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.