The crusades was a political thing. The kings just used god as a rally.
Many historians and some of those involved at the time, like Saint Bernard of Clairvaux, give equal precedence to other papal-sanctioned military campaigns undertaken for a variety of religious, economic, and political reasons
-Wikipedia (Source: Davies, Norman (1997). Europe – A History. Pimlico.)
And do you think an atheist world would be peaceful?
It would be about the same as normal, people killing people and doing horrific acts, but with proper consequences. None of this "Jesus wants me to kill dem niggers!" bullshit, and then people actually believing this cult to be truthful.