How do we know that baptism has to be immersion and nothing else, like sprinkling or pouring water on someone?

The New Testament was originally written in Koine Greek, i.e., the Greek spoken by the common man back in the first century A.D.  The Greek word for “baptize” or “baptism” is baptizo, which literally means to immerse.  “Baptize” or “baptism” are actually transliterations of baptizo, meaning that they are the result of taking the Greek term and changing its wording slightly to make it an English term without touching on what the term actually means.  Since the Greek term literally means to immerse someone, then whenever you read “baptize” or “baptism” in your English Bibles, just know that the original readers of the New Testament would have understood the Greek they were reading to mean to immerse or dip someone in water.