For if I pray ... - The reference to prayer here, and to singing in 1 Corinthians 14:15, is designed to illustrate the propriety of the general sentiment which he is defending, that public worship should be conducted in a language that would be intelligible to the people. However well meant it might be, or however the “heart” might be engaged in it, yet unless it was intelligible, and the understanding could join in it, it would be vain and profitless.
My spirit prayeth - The word spirit here (πνεῦμα pneuma) has been variously understood. Some have understood it of the Holy Spirit - the Spirit by which Paul says he was actuated. Others of the “spiritual gift,” or that spiritual influence by which he was endowed. Others of the mind itself. But it is probable that the word “spirit” refers to the “will;” or to the mind, as the seat of the affections and emotions; that is, to the heart, desires, or intentions. The word “spirit” is often used in the Scriptures as the seat of the affections, and emotions, and passions of various kinds; see Matthew 5:3, “Blessed are the poor in spirit;” Luke 10:21, “Jesus rejoiced in spirit.” So it is the seat of ardor or fervor Luke 1:17; Acts 18:25; Romans 12:11; of grief or indignation; Mark 3:12; John 11:33; John 13:21; Acts 17:16. It refers also to feelings, disposition, or temper of mind, in Luke 9:55; Romans 8:15. Here it refers, it seems to me. to the heart, the will, the disposition, the feelings, as contradistinguished from the understanding; and the sense is, “My feelings find utterance in prayer; my heart is engaged in devotion; my prayer will be acceptable to God, who looks upon the feelings of the heart, and I may have true enjoyment; but my understanding will be unfruitful, that is, will not profit others. What I say will not he understood by them; and of course, however much benefit I might derive from my devotions, yet they would be useless to others.”
But my understanding - (ὁ δὲ νοῦς μου ho de nous mou). My intellect, my mind; my mental efforts and operations.
Is unfruitful - Produces nothing that will be of advantage to them. It is like a barren tree; a tree that bears nothing that can be of benefit to others. They cannot understand what I say, and of course, they cannot be profited by what I utter.