But with respect to trine immersion in baptism, no truer answer can be given than what you have yourself felt to be right; namely that, where there is one faith, a diversity of usage does no harm to holy Church. Now we, in immersing thrice, signify the sacraments of the three days’ sepulture; so that, when the infant is a third time lifted out of the water, the resurrection after a space of three days may be expressed. Or, if any one should perhaps think that this is done out of veneration for the supreme Trinity, neither so is there any objection to immersing the person to be baptized in the water once, since, there being one substance in three subsistences, it cannot be in any way reprehensible to immerse the infant in baptism either thrice or once, seeing that by three immersions the Trinity of persons, and in one the singleness of the Divinity may be denoted. But, inasmuch as up to this time it has been the custom of heretics to immerse infants in baptism thrice, I am of opinion that this ought not to be done among you; lest, while they number the immersions, they should divide the Divinity, and while they continue to do as they have been used to do, they should boast of having got the better of our custom.
Gregory, Letter to Leander, Book I Letter 43