Why don’t all religions unite and fight against atheists since they are a common enemy for all religious people?
Good luck with that, OP.
Maybe you should try to step out of your immediate bubble, likely created by the preachers who are trying to keep the flock faithful by spreading misinformation about atheists, and come have a look here downunder in New Zealand at how our moderate Christians unite with us godless folks against the bigotry and intolerance of religious fundamentalists.
If you think that all religious people are on your side just because they are religious, then you need to have a look at what is happening in the real world. Most religious people in my country are secularists, humanists and anti-fundamentalists, who will oppose your intolerance for considering atheists as “the enemy”. Our moderate Christians are much more comfortable with the world view of secular humanists than with the world view of religious fundamentalists.
It is pretty much the same in Europe and Scandinavia.