ANSWER: It is an immersion in water! The baptism of the Holy Spirit (Matthew 3:11; John 14:15-26; John 15:26; John 16:7-14; Luke 24:49; Acts 2:1-4) was a promise made to the apostles (not a commandment) and fulfilled in Acts 2:1-4. A promise is not something to which a person can render obedience as demanded in the great commission of Matthew 28:18-20. Understanding this principle can help us to see which baptism is under discussion by an inspired writer as we study. Passages dealing with commandments to be baptized can always be taken to mean water baptism. For example, in Acts 2:36, the people to whom Peter was preaching asked, “what shall we do?” Peter commanded them. “Repent and be baptized” (vs.38). We can know then, without doubt, that this does not refer to Holy spirit baptism, because it is something people were commanded to do in order to gain the remission of sins (salvation). If the baptism here had signified Holy Spirit baptism, there would have been nothing for anyone to do! They would simply have received the promise!