Why does Paul say 'let him that glorieth glory in the LORD'?
Because God is the one using the man as His instrument. The person may be willing, but without the power of the Holy Spirit and the power of God, even Paul would accomplish nothing. Therefore glory belongs to the LORD, not to the instrument.
Les explains that the reason is that it is God who is using the man. God has an instrument that was willing, but without the power of the Holy Spirit and the power of God, even the apostle Paul would have accomplished nothing. He says this is basic for every believer—Sunday school teachers, those on television, and those teaching classes in church or home—because the glory belongs to the LORD, not the human instrument. He illustrates this by moving to Acts 16 and Lydia.