What makes Christianity unique is its attempt at universalism. Prior to it there weren't really belief systems that tried to make a claim to being both exclusiv

What makes Christianity unique is its attempt at universalism. Prior to it there weren't really belief systems that tried to make a claim to being both exclusively true and universal across all people. e.g. Judaism was/is explicitly for the Jewish people, the Romans freely borrowed from other pantheons and had no concern about other peoples worshiping whatever gods they wanted, etc. etc. Christians are the first major belief system we know of that declares not only that it is universal for all humans, but that if you don't believe/practice its specific faith you are damned, and that it is the duty of every Christian to convert others to the faith. This is one of the reasons why many in pre-Constantine Rome found it so objectionable and disruptive. Islam obviously followed in the same tradition. Various strands of Christianity go even further, making your practices almost entirely irrelevant and the inside of your thoughts being the key determinant on whether you are eternally damned or not. Ancient belief systems were very concerned about rituals and practices and sacrifices etc, vs e.g. strains of modern evangelical Protestantism that is obsessed with your feelings/thoughts/internal mental state.