I dunno, why not just have a new extremely aggressive (to other theists) religion that is a branch of the Abrahamic (for ease) which went through a huge state of religious 'conquest' before having it's shit slapped by the atheist/deist crowd bringing it to where it is now, a pro-"like, we're all just worshipping the same god guis, let's be friends" religion with "shit man, remember those guys? They were crusaders on steroids not so long ago" suspicious undertones for funsies.
Saves us having to bullshit a reason for all the religions to inexplicably merge and be besties.