In modern Christianity, baptism is a regular rite or "sacrament", practiced by almost all churches, although in a diversity of forms. But it was a new idea in John's day. The concept of baptism does not appear in the Old Testament. John was making a deliberately provocative statement.
John's concept of baptism was most likely linked to Jewish ritual washings and apparently practiced by those converting to Judaism. If so then John's baptism tells the Jews that they need this "conversion" as much as the Gentiles do! John treats the Jewish community as if they were pagan Gentiles!
This message is certainly consistent with John's emphasis on reform. In Matthew 3:9 and Luke 3:8 John tells the Jews, "Do not think... 'we have Abraham as our father'..." In emphasizing a conversion of the heart and mind, John explains that Jewish ethnic heritage is insufficient. The people of Israel need to fully embrace God (through their Judaism) with all the enthusiasm of a Gentile convert!
In modern times it is hard to look at old concepts (like baptism and communion) as they were seen in the first century, when they were new. In John's day, baptism was not a sign of "becoming Christian" but instead of sign of renewed commitment to Yahweh, the God of Israel. We should take a fresh look at rites like baptism and communion and see what they meant to the Jesus followers of the first century!
Wikipedia, of course, has a good article on baptism.
Tomorrow we will look at the beginning of the ministry of Jesus, as recorded by Luke just after the baptism of Jesus by John.