An atheist would probably say that we evolved from lower primates. Of course he would deny that God exists and he would probably say that after we die, we cease to exist. Since there is no absolute God, morals would not be absolute. Instead, they would be based upon personal preference and whatever works in society.
if we are created in God’s image, then we did not evolve from lower primates. This would mean that we have purpose and are not merely the result of random development through evolution that is, supposedly, guided by natural selection. Natural selection works on the theory of survival of the fittest, and this could have a very harmful effect on society if “survival of the fittest” is transferred into a moral principle. It would justify oppressing the weak and helpless.
Christianity influenced the development of the Constitution and American government. Our founding fathers developed the judicial, executive, and legislative branches of government which are there to exercise a system of checks and balances over each other. Why? Because of The Fall, man has a tendency to gravitate towards corruption.