It grew from the grassroots. Women and tax collectors and eunuchs and slaves and people on the wrong side of every political or religious or ethnic fence kept insisting. Peter and Paul and even Jesus gave it eloquent voice, but not until their everyday human hearts had been forcibly twisted around it by coincidences dramatic enough to get their attention, visions that would not go away, and people who would not give up. Universalism torn away from these roots in struggle and poetry pales to become something close to the vapid, indiscriminate relativism in which any one thing is as good as another. It has no sharp edges.
Hearts can wrap themselves around it with little threat of being opened very deeply.
Other may disagree, but I think the existence of denominations and rites within Christianity is evidence of its strength. It is not a simple religion, summed up succinctly and dealing with only a few issues. That's why it has exported so well through space and time. Schism, on the other hand, the splitting of groups that have acted as one in the past and have a chance and desire to continue that way, is an extravagance beyond most of our means these days, not only economically but intellectually. There is a limit to how small you can cut a piece of pie and still have anything resembling pie on the plate.