How could so many people that believe in the Bible and read the same books... Have so much divisions and conflicts between each other?

You are not the only one who have this problem, brother. Many of us have the same worry.
Why can't we be united as an army for Christ? I despite what conclusion we could draw from this, the problem is selfishness and personal interest. When we see ourselves as a family and count others better than us; when we truly love each other, there won't be such conflicts anymore.