Where did Alma get the idea to baptize the people? Mosiah 18:10

Where did Alma get the idea to baptize the people? Mosiah 18:10

Question rephrased:
Latter-day revelation tells us that Adam and Eve were baptized, and this passage in the Book of Mormon about baptism is also before Jesus Christ. But can we learn about baptism in non-latter-day saint sources that predate John the Baptist and the New Testament? Was baptism practiced and taught in Old Testament times?

Answer:
RESOUNDING YES.

It's not called "baptism" because that's an English version of a Greek word. And hey, none of the Old Testament was written in English originally anyway, so there's that, too.

But tons of scholars, ranging from Jewish to non-latter-day saint Christians to secular historians etc. concur that baptism really was a thing in Old Testament times.

This article references some of those Jewish etc. sources if you want to dig deeper.

Also, didn't they find something about this while excavating Qumron? In another life, one in which I wasn't a wife and mother of five kids, or in which the Czech Genealogy bug hadn't bitten me so squarely on the butt, I might be a linguist in an archaeological team in Israel, Palestine, or Egypt. That stuff is just so freaking amazing. Except that it's always political. So, yeah, no thanks. Why do people have to come along and suck so much, ruining everything deeply interesting?

Well, I guess that's why we need baptism...