Disunity is a problem on so many levels, between the races, between the sexes, between the colours, between the haves and have-nots, Russia and Ukraine, Labor and Liberal, Trump and Harris, North and the South.
When it comes to religion they have less of an excuse than any other group regarding disunity and yet you are right, religion leads people into rabid states of tribalism.
I don't deny what you say, you're sadly right, yet I still think religion is a core necessity that has got us to this point and will likely get us to the next point. I think humans are angry chimps without all the hair. We have a nature that is proving very difficult to tame and no matter what institution we enter we drag our instinctual tribalism and ignorance with us. Religion is just another opportunity to show we don't practice what we preach.
In many ways religion works like brainwashing, it hammers away with its illogical superstition and most people just want to be part of something, find a tribe to satisfy an essential need. The church offers a doctrine and culture that people accept without challenge, it has authority structures and this satisfies pecking order and protection that would have once been up to the young strong males in the tribe and it offers tribe and everybody needs their tribe.
The other thing to consider is that these religions are very old and there has never been a time when they have been more distant and lost to the origins of their foundations. Today these religions are the result of thousands of years of Chinese Whispers.
Basically there is no certainty greater than that held by the ignorant.