Earlier someone said water baptism is essential; no argument, but it is not entirely essential. God's grace knows no bounds; the good thief on the cross beside Jesus did not have the benefit of baptism.
What most do not realize is that water baptism did not originate with John the Baptist nor with Jesus. The people of Israel knew exactly what John was doing. The Pharisees did as well, but they did not know why.
“And they asked him, and said unto him, Why baptizest thou then, if thou be not that Christ, nor Elias, neither that prophet?” – John 1:25
The Pharisees knew that before the coming kingdom there would come a messenger who would clean the house of Israel in order for them to enter the ‘holy priesthood’ and take their rightful position as the ‘Priests of the Lord’
“But ye shall be named the Priests of the LORD: men shall call you the Ministers of our God: ye shall eat the riches of the Gentiles, and in their glory shall ye boast yourselves.” – Isaiah 61:6
Originally God excluded only the tribe of Levi to maintain the temple and only a select few of Israel were to be priests. However, it was always God’s intention to make the entire nation a nation of priests to all the world!
“Now therefore, if ye will obey my voice indeed, and keep my covenant, then ye shall be a peculiar treasure unto me above all people: for all the earth is mine: And ye shall be unto me a kingdom of priests, and an holy nation.” – Exodus 19:5-6
It was written in the Levitical law that the priests were to be first washed with water before entering their holy position.
“And Moses brought Aaron and his sons, and washed them with water.” – Lev 8:6
“And thou shalt bring Aaron and his sons unto the door of the tabernacle of the congregation, and wash them with water.” – Exodus 40:12
So then, John the Baptist came proclaiming the kingdom of God (the kingdom where Israel would be priests) and the remission of sins through water washing.
“John did baptize in the wilderness, and preach the baptism of repentance for the remission of sins.” – Mark 1:4
John preached his water baptism in the river Jordan for the remission of the sins. However, this water washing was not the only prerequisite for the priesthood.
“And thou shalt put upon Aaron the holy garments, and anoint him, and sanctify him; that he may minister unto me in the priest’s office.” – Exodus 40:13
Under the Levitical law the priests put on holy garments and were anointed with oil after their water baptism, but John the Baptist describes the true anointing with the Holy Ghost:
“I indeed baptize you with water unto repentance: but he that cometh after me is mightier than I, whose shoes I am not worthy to bear: he shall baptize you with the Holy Ghost, and with fire:” – Matthew 3:11
Both the water baptism and the Holy Ghost anointing were necessary requirements as described in the Levitical law in order for Israel to enter the kingdom.
In Luke 7:29-30 submission to water baptism was how believing Israel justified their faith in the coming kingdom as taught by Jesus.