The word baptism literally mean "immerse" or "immersion". Christ gave us an example...he was immersed in the river himself. There's much more than that. For example archaeologists digging close to Jerusalem’s Temple Mount have discovered nearly 100 ritual baths, or bathing pools, dating to the first century BC and the first century AD. This explains how the disciples baptized so many so quickly in Acts 2. The truth is there in scripture. The preponderance of evidence in the bible, along with jewish tradition and archoalogicial evidence, shows full immersion was the way people were baptized. Or at least how the first century church did it. Things start going off the rail soon after biblical times.
There was/is the Jewish ritual of “mikvah” which was why so many were found. It is STILL a practice. Almost every Jewish community has at least one. It didn’t start with Christianity.