These verses continue Paul's instruction to the Corinthian church about 'speaking in tongues'. There is a place for a personal devotional language in prayer and praise to God, but it is dangerous to be caught up in mystery and disengage the mind. In the church meeting, that will confuse others and let them think that relating with God is about feelings rather than objective truth.
 
Paul was always thinking about the enquirer, the unsaved person who might come to a church meeting. Their minds need to be engaged if they are to understand, consider and obey the gospel call. Conversion is the product of understanding the gospel, and choosing to repent of their sin and receive Christ. It is important for the believer too: true meditation on the Lord Jesus Christ is not through emptying the mind and having a spiritual experience, but by understanding and obeying the Word of truth.
 
As others are praying, we need to understand if we are to agree and make their prayer ours as well. Saying 'Amen' means – 'I agree', 'let it be so'. To affirm what you do not understand is foolish indeed. So the Apostle might pray in a tongue but ask the church to give the interpretation. He would also choose to pray in the known language, not only for the benefit of others but also that his mind might be fruitful. The same applies when singing songs of worship; sing words which will fill our minds with truth.
 
'Building up the church' is a repeated theme for the Apostle Paul. The church is the body of Christ. It is under attack by the world, the flesh and the devil; but Jesus is building His church and the opposition cannot stop Him. The believers should be helping and not hindering what Jesus is doing. Individually we need to be built up in God's love; together we need to grow up as God’s family. Selfish indulgence must not rule in the church, or it will be disconnected from its foundation, Jesus Christ (Colossians 2:19). So beware the unfruitful mind. Take time to read and pray and think; study God's Word with believing friends; learn to meditate on the Scriptures and let the Holy Spirit shape your Godward and earthly relationships.