Just because slavery is in the Bible does not mean God tolerates slavery, have you not read vengeance is mine says the LORD? (Isa 63:4, Romans 12:19) Yet God in his concern for all men addresses the fullness of His word even in slavery. New Testament texts, such as Colossians4:1, Galatians 3:28, and the Book of Philemon, make the case for the inherent spiritual worth of slaves, which effectively laid a base for deep down authentic change in social practice, over time. God's way is often to work from within, dealing primarily with the spiritual component.
But here is the really scary part. These things still exist and most people questioning the Bible are doing nothing to stop any of these things. I'm not fooled and neither should you be they question the Bible because it brings to light your own sin.
And in the end the only thing that matter is not what happened 3000 years ago, what matters is where you will spend eternity.
Your Brother in Christ