A couple of years ago I had the opportunity to talk with a Mandaean. These people are also called "John the Baptist Christians" and are a very old religion, sharing roots with Christianity and Rabbinic Judaism, but having doctrines unique to itself. They are few, clustered in ancestral communities near where the Tigris and Euphrates join as well as a Western Diaspora. They are particularly noted for devotion to John the Baptist, revering him as the greatest prophet and teacher.
Indeed, during his life, John the Baptist's influence was far-reaching. Like the other Prophets in the Tradition, he preached repentance, a radical departure from practice and mindset of the everyday world to embrace a life focused on the LORD and righteousness. Many esteemed him as a prophet and undertook his baptism to signal their commitment to that baptism (see today's Gospel reading, Matt. 3.1-12). Where Christianity departs from Judaism and Mandaeism is that John was more than a preacher or prophet, but the actual forerunner of Jesus, getting people ready for the root of Jesse prophesied in Isaiah (see today's Old Testament lesson, Is. 11.1-10). John preached repentance, but in this passage he stated that the one who is to come will baptise with the Holy Spirit and with fire.
This passage is about changing the rules of the game. Up until then the pattern was sin, repent, repeat, with a couple of judgements in between (like the Babylonian captivity, Antiochus IV Epiphanes, Rome...you name it). However, this passage is signaling a change in the pattern, where a more permanent, final approach is indicated, where God sends the Spirit not temporarily on select prophets, but on any who would follow Him, setting the world on fire in the process.
So here we are, listening to John on Jordan's bank today, looking beyond him to the figure of Jesus of Nazareth. Do we let the Spirit in and set us on fire to work for God's Kingdom, or do we just let ourselves burn to a worthless husk, of no significance whatsoever?