One Does Not Simply

It's harder to convince people you exist if they already have a god(s)

It's especially hard if one of your former angels is down on the world and trying to make your creations forget you even exist.

If all things are possible with God, then he could have contacted people of different cultures and taught them to get along instead of remaining silent while different religions fought each other.

The Crusades were started by followers of the Roman Church, and as we've seen in our history countless times they're not above going against God's will and changing his law, also something he said that was impossible. And as I mentioned Satan is down here on earth deceiving people, so even if God did tell them "Hey stop being idiots" Satan would get them back to their ways at some point.

It wouldn't be hard to find Bible quotes that say that the only reason Satan is allegedly causing trouble is because God let's him including ones from the Books of Job and the Apocalypse.

Also god could have told Protestants, Muslims, Mormons or other denominations that these religious wars aren't really such a great idea.

Even if God had the opposing religions would of still have attacked at some point, and God lets him because destroying him wouldn't fix much of the problem of sin we have.

The apocalypse is really only deadly for those with the M.O.T.B. (which is symbolic, not literal).