Paul and the apostles preached the New Testament, declaring the spirit of the word from what we call the Old Testament. We know why. That's the pattern our Lord gave to us. In John 3, he took the letter of the word from Numbers 21 regarding the serpent lifted up, to declare the Spirit of the word to Nicodemus. Showing us the accomplished redemption of Christ from the letter of the word. He gives us this as a pattern of which we are to follow in all the word of God: To minister the Spirit and Grace of God in Christ to the people. We are to do that rather than focusing on the letter of the word, which only ministers death and condemnation.