Romans 12:19

Let's join the crowds trudging out toward the Dead Sea, listen with me at the back of this massive group of people listening as John the Baptist preaches. They have come into the region to the immediate West of the Dead Sea - an utterly barren desert. This area was known as the realm of the Jewish sect of the Essenes, and they had significant communities in this region. Of course, there is no biblical evidence to suggest that John was in any way connected with that sect. John also seems to have preached near the northern end of this region, close by where the Jordan flows into the Dead Sea, rather than midway down the shore.
John was the last of the old, as well as a messenger of the new. As he stood thundering the call to repentance, he pointed to the Lamb of God who by one sacrifice would pay the sufficient price for the sin of the world. His powerful words are recorded by God, through the eyes and ears of Peter, by the hand of Mark, and under the inspiration of God, and can be found in Mark 1:4-8.
What was John's message?
It was simply, Repent. This is no mere academic change of mind, nor mere regret or remorse. John the Baptist spoke of repentance as a radical turning from sin that inevitably became manifest in the fruit of righteousness. The symbolism of John’s baptism likely had its roots in OT purification rituals (Leviticus 15:13). Baptism had also long been administered to Gentile proselytes coming into Judaism. The baptism of John thus powerfully and dramatically symbolized repentance. Jews accepting John’s baptism were admitting they had been as Gentiles and needed to become the people of God genuinely, inwardly (an amazing admission, given their hatred of Gentiles).