Baptism: It’s Pagan, Guys. Get Over It. • Richard Carrier Blogs

…"The features Christians lifted from the pagan mysteries are: passing from one state to another—life, through death, to a new life, symbolically through the water ritual. The features Christians lifted from Jewish water rituals are: humble simplicity, a joining to the body of Israel, and an atoning function that secures one access to the future resurrection. 

The feature both, Jewish and pagan, already shared, was this feature of cleansing sins in preparation to receive this new state and benefit. Christians used that common nexus to build a hybrid ritual, combining their preferred elements of both, to realize their own form of baptism. And when this began, they brought with it even such particulars as baptism for the dead, demonstrating the link to its pagan cultural past, and added joining "the body of Christ" as an analogy to joining "the body of Israel" in Jewish conversion-initiation water rituals… 

Following pagan precedent, the rite was conceptualized as a symbolic but metaphysically potent death and resurrection. And following precedents in both Judaism and paganism, this was all tied to a cleansing away of past sins. 

Baptism was thus no more unique to Christianity than a resurrected or virgin born savior…"

https://www.richardcarrier.info/archives/30551