I think it’s important for people to remember, especially Christians to remember, that baptism had already existed in a different form, or in a similar form, prior to what we know today as baptism. When John the Baptist is in the river baptizing people, people didn’t walk by and say, “What’s he doing? That’s a strange thing!” They knew what he was doing, he was baptizing; and this probably emerges from the idea of ritual immersion that existed in Judaism long before. We have evidence of ritual immersion going on prior to the advent of Christianity because we have mikva’ot (mikvehs) we have Jewish ritual immersion and this was for ritual purity. Before you could go worship, you would ritually immerse yourself and this wasn’t necessarily for hygiene or for cleanliness, it was for spiritual purity. You would make yourself pure so that you could go in the presence of the deity, you could worship, you could offer sacrifices. So, long before Christianity developed the idea of baptism, Jews had been practicing ritual immersion in the form of entering into a mikveh, immersing ones selves and then emerging from that purified.
Judaism was incredibly diverse in the Second Temple period, in this period after returning from exile, and then of course, leading up to the rise of Christianity. We know of some groups that practice ritual immersion; we also have physical evidence, we have actually have mikva’ot that are discovered in and around Jerusalem and in a place called Qumran, associated with the discovery of the Dead Sea Scrolls. We know that certain ancient authors like Josephus talk about groups like the Essenes who would practice ritual immersion, regular ritual immersion.. And so we actually have textual sources and archaeological evidence that demonstrates that certain groups were obsessed with ritual purification.
The Christian approach to baptism was like many other approaches to Jewish rituals; for instance, Jesus took the Passover meal and invested new meaning upon it, you know, “this is my blood, this is my body” and gave it some new meaning even though it was an established Jewish practice, the Passover meal. In the same way, Christians looked at ritual immersion and invested a new meaning upon it. So, instead of a regular ritual immersion that’s done every time that you would go up and worship, the Christians took this same practice of physical immersion into a body of water for the purposes of spiritual purification and adopted this as baptism. The idea being, though, that the Christians would do it one time, that you would be baptized once. And, there are early documents that didn’t make it into the Bible that talk about how you don’t need to be baptized three or four times, just baptized once and it became a form of initiation, of designation that I am now giving myself over to Jesus and to the Christian faith.