Might want to read that Bible again, sometime. Just because people did things like take multiple wives and concubines doesn't mean it was ok in God's eyes. In every instance where a man had more than one wife, he suffered the consequences of that sin. The Bible shows us lots of people doing lots of dumb things and having to deal with the consequences. We wouldn't learn much from a book full of perfect people, now would we?
And throughout history, just because human beings at heads of various "Christian" sects did dumb things doesn't make them right either. Lots of people call themselves Christians who aren't
Feel free to be a non-believer, but might be best to refrain from speaking to the veracity of something you don't know or understand