Luke 3:3 “He went into all the country around the Jordan, preaching a baptism of repentance for the forgiveness of sins.”
Israel’s freedom came because they left Egypt, crossed the Red Sea, died in the wilderness, crossed the Jordan and entered the Promised Land.
They are now in slavery again and this was John’s re-enactment, a new Exodus, not a fleeing to a Promised Land but a return to God.
The Mikveh was common to the people and is still practiced by Jews today. The ritual baths outside the temples helped worshippers rid themselves of the inner dirt that they had become contaminated by prior to them worshipping Jehovah. It symbolised confession of sin with a hope of forgiveness. It also indicated that the person was walking from the old into the new. It was a common and regular practice. There was nothing unusual.
Except that is until John comes along.
Granted the river Jordan would be seen as an unusual Mikveh, but where is the temple? This was not for John about worship
John had brought the people back to the Jordan where they had gone through to their Promised Land to show them that they were still after all these years in need not of the Mosaic Law with all of its 613 commandments which they couldn’t keep, but in need of a Redeemer! They were still needing a Moses figure.
The Law cannot do it. The Redeemer can.
Stop trying to satisfy and fulfil and do in order to be acceptable rather come away from the old life, from your Egypt, through the Mikveh.