I usually take a slightly different tack on the baptism of Jesus. For one thing, a Baptist might note that the paedo argument advocated below has difficulty with the pronouns.
Who looked up and saw the Spirit descending on
him? The pronouns are ambiguous. However, given the fact that the second pronoun must refer to Jesus, and given the fact that Jesus was the subject of the passive and active verbs in verse 9, it seems rather conclusive to me that
Jesus is the One who saw the heavens opened, and Jesus was the One upon whom the Spirit descended. However, the passage is completely inconclusive with regard to immersion, given the geography of the Jordan River, which has extremely steep banks leading to a rather shallow river. One would have to be standing in the river to make use of the water in the river
at all. So "coming up from the water" could just as easily mean "got out of the Jordan" as meaning "came up from underneath the water." If they were coming up out of the Jordan River, then the baptism is done (though just done: "euthus"), and as they climb out of the Jordan, Jesus sees the heavens open and the Spirit descend upon Him.
