I doubt that the Gospels are showing us any single occasion of John’s preaching; how could they be, since they are worded differently? In each book I think we are being shown different chapters from The Best of Rev. the-Baptist, a John-the-Baptist casserole, designed not to Memorex a particular sermon but to give us a feel for what his message was. In general, his message was probably, I’ll baptize you with water, yes, but just you wait until He comes, and then He’ll baptize you with the Holy Spirit!

I think it seems clear that John’s talk of the baptism of fire in Matthew and Luke is an occasion (or occasions) in which he addressed the Jewish leaders, specifically referring to the burning of the chaff that occurred with the end of the Old Covenant. I think we have falsely conflated baptism with the Holy Spirit and baptism with fire because of “what seemed to be tongues of fire” [my emphasis] resting placidly on the heads of the partakers of the New Covenant at Pentecost. This may have been a sign that they, like the burning bush, were not consumed by the judgment of fire and were hence vetted by God as exempt from judgment. Of course, it could also have been unrelated; the upper room phenomenon did just look like fire, after all. I just find it too incredible that John ties baptism with fire and the burning of the chaff so closely together if they are not referring to the same thing.