God makes it very clear that the two covenants are not to be mixed: “Cast out the bondwoman and her son” (Gal 4:30)… The old covenant operated by principles, the new covenant by promise… The distinction between principles and promise also explains why the Apostle Paul chose not to preach anything to the Corinthians other than “Christ, and him crucified” (1 Cor 2:2). That puzzled me somewhat as I thought of all the wonderful Old Testament truths that were readily available to Paul. He could easily have preached Proverbs with all its wisdom. He could have preached Psalms with its varied and creative expressions of intimacy. Or he could have explored the many lessons learned from the history of Israel. Yet he focused completely on his message on Christ crucified. Why? Because Christ’s death and resurrection represent the highest and most climactic revelation of God’s entire design. It is God’s bringing life out of death and something out of nothing according to his purpose. That really is gospel!
Grace Works, Vine Books, 1992, 128-9.