Pursue [this] love [with eagerness, make it your goal], yet earnestly desire and cultivate the spiritual gifts [to be used by believers for the benefit of the church], but especially that you may[a]prophesy [to foretell the future, to speak a new message from God to the people]. For one who speaks in an unknown tongue does not speak to people but to God; for no one understands him or catches his meaning, but by the Spirit he speaks mysteries [secret truths, hidden things]. But [on the other hand] the one who prophesies speaks to people for edification [to promote their spiritual growth] and [speaks words of] encouragement [to uphold and advise them concerning the matters of God] and [speaks words of] consolation [to compassionately comfort them]. One who speaks in a tongue edifies himself; but one who prophesies edifies the church [promotes growth in spiritual wisdom, devotion, holiness, and joy]. Now I wish that all of you spoke in unknown tongues, but even more [I wish] that you would prophesy. The one who prophesies is greater [and more useful] than the one who speaks in tongues, unless he translates or explains [what he says], so that the church may [b]be edified [instructed, improved, strengthened]. – 1 Corinthians 14:1-5
By the aid of the Spirit, we should, in fact, deliver the most important and sublime truths. This would doubtless be the case, that those who were thus endowed would deliver most important truths, but they would be “lost” upon those who heard them, because they could not understand them. The phrase “in the Spirit,” evidently means “by the Holy Spirit,” that is, by His aid and influence. Though we should be “really” under the influence of the Holy Spirit, and though the important truth which we deliver should be imparted by His aid, yet all would be valueless unless it were understood by the church. – Barnes
Though our own mind (for so πνευματι is understood here by many eminent critics) apprehends the mysteries contained in the words which we utter; if, by the spirit, we understand the Spirit of God, it only shows that it is by that Spirit that we are enabled to speak and apprehend these mysteries. – Adam Clarke
Whoever speaks in an unknown tongue speaks to God. No one understands it, and in the Spirit that person speaks mysteries.
Vine Expository Dictionary – mysteries:– that which, being outside the range of unassisted natural apprehension, can be made known only by divine revelation, and is made known … to those only who are illumined by the Spirit.
Words are uttered thoughts, so when we speak in tongues as the Holy Spirit gives utterance, we are speaking God’s thoughts.
Through speaking in tongues, the Holy Spirit is able to reveal to us the hidden truths about God and His way of doing things.
Praying in tongues is a gift for communicating with God.