Which religion is found in every culture, every country, even the cultures that will kill the adherents...?

I don't think the religion you seek exists or will ever exist. A religion is just an organized way of worshipping God. They all worship the same Creator. If we all exist in the same reality than our God is the same for everything in our reality. I like to incorporate the concepts of all religions in my philosophy. All religions have elements that I disagree with, so I will not commit myself to a system of worship I don't see eye to eye with. God is the creator of reality and transcends our systems of religion.