Perhaps the fact that the OT was written to a people who, although were God's chosen people, were not ready for the teachings of Christ. They were taught the letter of the law but not the spirit of the law. That may explain a bit. - TTWSYF
God instructed his people to take slaves as possessions. He invited them to kidnap virgins as sex slaves. He ordered his people to hack thousands of women and babies to death in cold blood. That is the letter of the law, god's law. The law of the god and father of Jesus.
Are you really suggesting that god's chosen people were not ready to be told not to murder babies wholesale and rape virgins? If god had said, "you will not take another human being as a possession" would it really have been too much for them to comprehend? That is a bizarre explanation.
You have no option but to declare rape, slavery and infanticide to be acts of perfect morality. That is the inevitable consequence of appealing to god as the ultimate standard of objective morality.
I certainly would not expect an atheist to understand the diff.
I was a christian for almost a decade after I left the cult. I understand very well.
These are reasons why Christianity is true and atheism is false and never gave anything good to the people of the world.
You have all your work ahead of you to explain why christianity is true. You might persuade me it is useful but certainly not true. Atheism is nothing more than the honest confession that the claims of christianity are unconvincing. Science, rationality and humanism is responsible for all of the progress in society since the Enlightenment. Religion had centuries and failed. The hungry are being fed, the sick are being cured and its all thanks to abandoning old superstitions.
I refuse to approve of rape, slavery and infanticide. You have no such option without condemning your god.