John Calvin Commentary 1 Corinthians 14:15

"What is it then? I will pray with the spirit, and I will pray with the understanding also: I will sing with the spirit, and I will sing with the understanding also." — 1 Corinthians 14:15 (ASV)

I will pray with the spirit. So that no one should ask, by way of objection, “Will the spirit then be useless in prayer?” he teaches that it is indeed lawful to pray with the spirit, provided the mind is also employed, that is, the understanding. He allows, therefore, and sanctions the use of a spiritual gift in prayer, but requires, as the main thing, that the mind is not unemployed.

When he says, I will sing Psalms, or, I will sing, he uses a particular instance instead of a general statement. For, as the praises of God were the subject matter of the Psalms, he means by the singing of Psalms—blessing God, or rendering thanks to him; for in our supplications, we either ask something from God, or we acknowledge some blessing that has been bestowed on us. From this passage, however, we also infer that the custom of singing was, even at that time, in use among believers, as also appears from Pliny, who, writing at least forty years, or around that time, after the death of Paul, mentions that the Christians were accustomed to sing Psalms to Christ before dawn. I also have no doubt that, from the very first, they followed the custom of the Jewish Church in singing Psalms.