He gave orders to stop the chariot. Then both Philip and the official went down into the water. Philip baptized him. (Acts 8:38 NIrV)
And when you were baptized, it was the same as being buried with Christ. Then you were raised to life because you had faith in the power of God, who raised Christ from death. (Col 2:12 CEV)
We were therefore buried with him through baptism into death in order that, just as Christ was raised from the dead through the glory of the Father, we too may live a new life. (Romans 6:4 TNIV)
This question arises all the time with the discussion about baptism. "I was baptized as a baby and they poured water over my head." "I had water sprinkled on me when I was baptized." "I was baptized by being dunked." Are all these forms the same?
The Greek word, βαπτίζω (baptizo) simply means "to dip, to immerse, or to submerge." When a Greek-speaking person of that day would dye a piece of cloth, they would "bapto" it in a tub of dye. That way the entire cloth would change to the desired color.
The only Scriptural examples of water baptism (like the ones above) refer to going "down into" the water or "being buried" in the water. This includes John's baptisms, Jesus being baptized, Jesus' disciples baptizing people as followers of Jesus, and the baptisms occurring after Jesus was raised and ascended.
No other variation on the theme occurred until later in church history. These variations appeared in writings first to accommodate death-bed conversions, then later because of the fear that unbaptized infants may go to hell if they are not baptized. None of these examples or accommodations took place in Biblical days.
What is baptism, then? Simply going "down into" the water.