Matthew 3 is the first mention in the Bible of baptism: John came baptizing.
I like how baptism is an assumed thing. Like this wasn’t weird to anyone. John just comes baptizing and people are like, “Oh, yeah, well of course, guess we should go be baptized, eh?” He didn’t explain why this occurred. It just made sense to the people.
This is probably from an OT understanding of purity and cleansing.
When an Israelite was unclean for any number of reasons, part of the remedy was to bathe in water (Leviticus 14:8-9; 15:13; 17:15). Utensils used in tabernacle and temple service were to be dipped in water to be cleansed.
In Numbers 19:9 the ashes of the heifer “are to be kept by the Israelite community for use in the water of cleansing; it is for purification from sin.” The whole chapter of Numbers 19 is filled with “water of cleansing” talk.
Ezekiel 36:25 says, “I will sprinkle clean water on you, and you will be clean; I will cleanse you from all your impurities and from all your idols.”
The people of Israel knew that water meant cleansing, and it was stated specifically several times that it was for removing sin.
When John pops on the scene, preparing the way for the Messiah, and announces people should repent and be dunked in water, this just made sense.
John didn’t explain why baptism was a thing. It naturally flowed from all that had proceeded in the Bible about water, sin, and cleansing.
There is a clear connection with water and the cleansing of sin in the Old Testament.
It should also be noted that in the Creation Account in Genesis 1, the Spirit hovered over the face of the deep, over the water. The first mention of the Spirit in the Bible is in connection with water.
The connection is explicitly made in passages like Isaiah 44:3, “For I will pour water on him who is thirsty, and floods on the dry ground; I will pour My Spirit on your descendants, And My blessing on your offspring.”
Again we see water in connection with cleansing from sin and also something to do with the Holy Spirit, just like in Matthew 3 where baptism is described in these two ways.
Coming out of the OT, baptism seemed a natural and understood concept. The Jewish people got what John was doing. It wasn’t complicated. It shouldn’t be for us either!