If Christ is God, then wouldn't all the teachings of God in the Old Testament be the teachings of Christ?
Did God change?
God doesn’t change, but we (as in Mankind) do. Human history is a story of progression. It is, frankly, an uneven progression with lots of reverses over the centuries, but socially things have progressed. What God is doing is slowly bringing us to a more complete understanding of Himself and His will.
Consider. If your three year old punches his brother and steals his candy, you don’t give him a treatise on advanced ethics, true though all that would be. Not because it isnt right, but because said three-year old wouldnt understand it. You take more immediate, and less academic, action. When he grows up he can get a treatise on the rights of man