The following is an excerpt from The MacArthur New Testament Commentary on Acts 8 .

And as they went along the road they came to some water; and the eunuch said, “Look! Water! What prevents me from being baptized?” (Acts 8:36)

At some point as they went along the road the eunuch was granted saving faith and was instructed about baptism. That is implied by his reaction when they came to some water. The eunuch said to Philip, “Look! Water! What prevents me from being baptized?” They came across a pool or stream in the desert at just the appropriate moment for the man to publicly testify to his saving faith by being obedient to the ordinance of immersion. That is yet another example of the sovereign Spirit’s control of events.

[And Philip said, “If you believe with all your heart, you may.” And he answered and said, “I believe that Jesus Christ is the Son of God.”] And he ordered the chariot to stop; and they both went down into the water, Philip as well as the eunuch; and he baptized him. (8:37–38)

The oldest and most reliable manuscripts do not contain verse 37, which should be omitted from the text. Still, something like that confession must have occurred. Coming to the water, the eunuch ordered the chariot to stop; and they both went down into the water, Philip as well as the eunuch; and he baptized him. As noted in the discussion of Acts 2:38 in chapter 6, baptism is the public confession of faith expected of all believers. The eunuch not only confessed his faith personally to Philip but openly in front of his entire entourage. That both he and Philip went down into the water indicates that his baptism was by immersion. So does the word baptizo, which means “to dip” or “to immerse.”