Why has Christianity spread widely despite persecution?

Christianity has spread widely since its inception in a very short period. Despite the persecutions from the rulers and representatives of pagan religions, it spread at a rapid pace. It was in many ways the religion of the poor and slaves, beggars and disadvantaged, who constituted the overwhelming majority of society at that time. Christianity was successful with them because it preached such values as mercy, kindness, forgiveness, purity of morals.