What does it mean to pray in the spirit?
1 Corinthians 14:13-16 13 Therefore let him who speaks in a tongue pray that he may interpret. 14 For if I pray in a tongue, my spirit prays, but my understanding is unfruitful. 15 What is the conclusion then? I will pray with the spirit, and I will also pray with the understanding. I will sing with the spirit, and I will also sing with the understanding. 16 Otherwise, if you bless with the spirit, how will he who occupies the place of the uninformed say "Amen" at your giving of thanks, since he does not understand what you say?
It is helpful to understand the Pharisaical doctrine of, “Praying in the Spirit.” It was widely taught and still is today that the only acceptable way to pray to God is to pray in Hebrew. This is considered to be the divine language and everyone who aspired to be a good Jew would learn how to pray in Hebrew. There is a huge difference between praying in Hebrew and understanding what you’re praying. To this day, it is very unusual to find a Jew in a synagogue who cannot pronounce the words, but it is also more unusual to find a Jew who knows the meaning of the words he is pronouncing. Learning the meaning of the language is simply not part of most synagogues. The Pharisees taught then, and most Jews still teach today that praying in Hebrew, the language that God understands and speaks is, “Praying in the Spirit.” You are communicating to the Holy Spirit through your spirit by praying in the divine language, regardless of whether or not you understand what you’re praying. The prayers were prewritten for you by the religious authorities.
If you keep in mind that Paul was dealing with many Corinthians that were first evangelized by the Pharisees, you should expect that he would need to address one of the most important teachings that would have been perpetuated. To maintain order in a public assembly, if a person wants to speak to the group in a different language, they should also have the courtesy to translate what they are saying so everyone can understand. This is especially true if the person is praying on behalf of the congregation. Understand that this is a major issue that would cause a complete reorganization of how meetings were being held.
Paul’s conclusion was that he was going to pray in a language he understood and also sing in a language he understands. The songs were arranged and sung in Hebrew just as the prayers were. This would be motivated from the core of his spirit, but certainly not at the expense of having a clear mind of exactly what is being communicated.