The word baptize is only found in the New Testament of the Bible. Prior to John the Baptist preforming this water ceremony, it is never mentioned. John, by the way, baptizes Jesus.
The Old Testament doesn’t mention baptism and there is no biblical account of its origin. It seems to have just started on its own, beginning with John the Baptist. Did John invent it? Perhaps God told John to do this new thing, pointing people to a new way – Jesus.
I don’t know the answers to these questions, even though I ponder them a lot. And I can’t find much of a clue in the Bible. Though some people attempt to connect New Testament baptism with Old Testament uses of water in religious ceremonies and rituals, I think any correlation is weak.
The dictionary describes baptism using the words cleanse, purify, and initiate. This helps some, since the first few books of the Old Testament talk a lot about cleansing and purification. Yet pulling the ceremony of baptism from them seems a stretch.
However, in a curious passage in Corinthians, Paul talks about the Old Testament Israelites being “baptized into Moses.” Since I can’t find an actual Old Testament account of this happening, I assume it is a figurative baptism, not actual.
None of this, however, gets me any closer to learning the basis for baptism. But what’s important to know is that Jesus tells us to do it.