4 John appeared, baptizing in the wilderness and proclaiming a baptism of repentance jfor the forgiveness of sins. 5 And all the country of Judea and all Jerusalem were going out to him and were being baptized by him in the river Jordan, confessing their sins. 6
Ceremonial washing was familiar within the world of 2nd temple Judaism. This ceremonial washing was a repeated act. Within Judaism a once for all time immersion was required for a convert to Judaism. A sign along with circumcision that the gentile was now a full member of the covenant family. John offers a baptism which in many respects is similar to the experience of the proselyte. He is calling the people to come and join a new movement. He preached a baptism of repentance (μετανοίας), a call for people to change their mind, their worldview. To embrace something different and leave certain attitudes and lifestyles aside. This ‘changing of mind’ baptism would bring the results of the forgiveness of sins. 2nd temple Judaism, commanded by YHWH in the Tanack (Old Testament) did provide through the temple cult forgiveness of sins. John here is offering a forgives outside of the temple cult.
Judaism at the time of John saw itself as in some sense still being under the judgement of God. This judgement which came about during the exile was still taking place. Yes the people had returned to the land but the great promises of Isaiah (see verse 2) had not been fulfilled. The pagan rulers were still in charge, YHWH had not returned to Zion, the sin which had led to exile had not been forgiven. Forgiveness of sins is not just 1:1 between man and God. It is that, but more than that. It’s a restoration of Israel, a restoration of a humanity to what God intended them to be.
In the 21st century we need to see our personal forgiveness as an opportunity to repent/change our worldviews in anticaption of the reign of God. The exile is over, YHWH’s presence is now with his people,
John stands at the Jordan, the border lien to the Promised Land. He encourages people to change their minds, to become the real people of God. The result would be the arrival of the reign of YHWH, the kingdom of God.