As best as I can understand it, the theme of the Old Testament is that God created mankind and placed us on this earth with a great purpose in mind. There are hints, but only hints, that part of that purpose was that we would be a testimony of His love and kindness to the angels that had rebelled. Many times God appeared on earth, in human or angelic form, and talked with those who were endeavouring to accomplish His purpose. He guided and supported them with many manifestations of His power and glory. But in the end it was evident that mankind was not strong enough, or wise enough, to do what God expects of us.
The theme of the New Testament is that God never expected us to do it in our own strength and wisdom. In the Sermon on the Mount, Jesus taught us a way of life that looks a lot like weakness. Jesus Himself submitted to the outrages of those who were powerful according to the measure of this world, and overthrew them by the power of love and forgiveness. The apostle Paul saw how the power of God could only work in us when we are weak, and said, “Therefore I take pleasure in infirmities, in reproaches, in necessities, in persecutions, in distresses for Christ’s sake: for when I am weak, then am I strong.”
We often speak of the armour of God as described by Paul in Ephesians chapter six. Do we realize that this armour is only effective when we lay down our own armour? Isn’t this the significance of David taking off Saul’s armour and laying it aside before he went out to meet the giant? He told Goliath, “Thou comest to me with a sword, and with a spear, and with a shield: but I come to thee in the name of the LORD of hosts, the God of the armies of Israel, whom thou hast defied. This day will the LORD deliver thee into mine hand; and I will smite thee, and take thine head from thee; and I will give the carcases of the host of the Philistines this day unto the fowls of the air, and to the wild beasts of the earth; that all the earth may know that there is a God in Israel.”
Goliath saw nothing but a sling in David’s hand, but David was clad with the whole armour of God, invisible to human eyes, yet more powerful than any weapon of human warfare. We are powerless against the giants of our day if we face them in our own strength and expect God to provide a little supplementary force. The message of the New Testament is that we must put off every vestige of human strength and trust only in God’s strength.