1 Corinthians 14:15

15.I will pray with the spirit Lest any one should ask, by way of objection, “Will the spirit then be useless in prayer?” he teaches, that it is lawful, indeed, to pray with the spirit, provided the mind be at the same time employed, that is, the understanding He allows, therefore, and sanctions the use of a spiritual gift in prayer, but requires, what is the main thing, that the mind be not unemployed. (834)

When he says, I will sing Psalms, or, I will sing, he makes use of 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 (835) — 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 conferred upon us. From this passage, however, we at the same time infer, that the custom of singing was, even at that time, in use among believers, as appears, also, from Pliny, who, writing at least forty years, or thereabouts, after the death of Paul, mentions, that the Christians were accustomed to sing Psalms to Christ before day-break. (836) I have also no doubt, that, from the very first, they followed the custom of the Jewish Church in singing Psalms.