The Seal of the gift of the Holy Ghost.But Eunomians, who are baptized with only one immersion, and Montanists, who are here called Phrygians, and Sabellians, who teach the identity of Father and Son, and do sundry other mischievous things, and [the partisans of] all other heresies— for there are many such here, particularly among those who come from the country of the Galatians:— all these, when they desire to turn to orthodoxy, we receive as heathen. On the first day we make them Christians; on the second, catechumens; on the third, we exorcise them by breathing thrice in their face and ears; and thus we instruct them and oblige them to spend some time in the Church, and to hear the Scriptures; and then we baptize them. (Canon 5)
Eunomius (circa 360) introduced single immersion “into the death of Christ.” This innovation was condemned. Apostolical Constitutions, 50, says, “If any presbyter or bishop does not perform the one initiation with three immersions, but with giving one immersion only into the death of the Lord, let him be deposed.” Single immersion was allowed by Gregory the Great (circa 691) to the church in Spain in opposition to the Arians who used a trine (not triune) immersion (Epis., i. 43). This was exceptional.