1 Corinthians 14:14

For if I pray in an unknown tongue
In the Hebrew tongue, which the greatest part of the Jewish doctors insisted ^F1 upon should be only used in prayer; which notion might be borrowed from them, and now greatly prevailed in the church at Corinth; and the custom was used by such as had the gift of speaking that language, even though the body and bulk of the people understood it not:

my spirit prayeth;
I pray with my breath vocally; or else with affection and devotion, understanding what I say myself, and so am edified; or rather with the gift of the Spirit bestowed on me:

but my understanding is unfruitful;
that is, what I say with understanding to myself is unprofitable to others, not being understood by them.