Good post, but I think it would be truer to say that Christian leaders co-opted pagan rituals in order to sell the new religion of the state to the people of the Roman empire, who were well grounded into their pagan rituals and beliefs. It's a terrible testimony of the church, in my opinion. Constantine may have simply been playing politics, realizing this upstart religion was gaining new converts faster than the Roman empire could spread, so, instead of fighting them, or persecuting them, let's make their religion the religion of the state, then we can slowly paganize them without them knowing it. I don't know if that was his intent or not, but it worked. Later, the emperor Julian would attempt to take Rome back to its pagan roots, unsuccessfully.

While the rituals of the pagan religions were incorporated into Christian worship, the belief systems--the core of the religions themselves--are worlds apart. Christian philosophy and ethics is rooted in Judaism, which was adamantly not pagan.