I wouldn't use the words "rip-off". That would imply that they conciously copied off of pagan mythology. They didn't- it was probably much more on a subconscious level, although the motifs passed along from civilization to civilization are undoubtedly of pagan origin.
What I find hilarious is, as much downtalking to paganism as Christianity does, walk into a catholic church. You will see enthroned idols, small devotional shrines, elaborately garbed priests, sophisticated altars, and occasionally even burning incense! Not far off from the pagan temples of old. Nothing in the basics of religious devotion has changed in the past 2,000 years, except maybe for the certain lack of animal sacrifices nowadays. Even a stark "puritan" church, with a simple wooden cross behind the pulpit, could be accused of idolatry.
Christianity cannot possibly separate itself from paganism, as the concepts of a reincarnated deity, communal sacrifice, divine impregnation of a virgin, the idea of "God as three unified but distinct persons", etc, all reek of heathen religion and polytheism, and yet they are embraced by even the most sterile of puritans.
«Recibe, ˇoh Lucifer! la sangre de esta víctima que sacrifico en tu honor.»