Today there is much debating regarding various views on baptism. Scripture tells us to not add to it or take away from it and here in Luke we are given a definition of what John was teaching about baptism.
Verse three, John was ; "...preaching that people should be baptized TO SHOW that they had repented of their sins AND TURNED to God to be forgiven." (capitals emphasis mine)
Verse three clearly tells why people should be baptized.
1. To show they had repented of sins
2. To show they had turned to God for forgiveness of their sins
Luke gives us more information about John and baptism in verse seven. People came to him to be baptized but he lectured them about coming with a wrong heart. They came to him more or less thinking baptism would protect them from God's wrath. John must have understood they had not changed the way they lived but only wanted a ticket to forgiveness and called them a brood of snakes.
Verse eight, John spoke to them; "Prove by the way you live that you have repented of your sins and turned to God." The act of baptism was to reflect their turning away from sin and that they were looking to God for forgiveness. He was telling them that the sincerity of their baptism was expressed by how they lived from that day forward. Being baptized should mark the point in one's life where they live a more Godly lifestyle. Some thought they were safe because they were descendants of Abraham and John helped them understand that didn't matter in this case.
Hearing that, the people asked what they should do. John replied in verse 10, "If you have two shirts, give one to the poor. If you have food, share it with those who are hungry." Even the tax collectors asked what they should do and John spoke to them in verse 13, "Collect no more taxes that the government requires." (Most tax collectors charged the people more that the government asked of them and kept what was left over for themselves.)
Even some soldiers asked what they should do and John spoke to them saying, "Don't extort money or make false accusations. And be content with your pay." Soldiers could have people pay them for protection or pay to have the soldiers overlook some unlawful activity or business. They could be bought in many ways adding extra income to their pay.
The point here is that John instructs people to be generous, to share, to care for those that are struggling or in need. He teaches us to not take advantage of others for personal gain. He teaches we should be people of integrity, uncompromising in doing what it right. Simply, to love and care for one another and to be people of honor who do the right thing no matter what.
Some wondered if John was the Messiah. He clearly stated was not. He added he was not good enough to be a slave of the Messiah nor to untie the Messiah's sandals. John added that he has been baptizing with water (verse 16) while the Messiah would baptize with the Holy Spirit and fire.
John's water baptism was the way people expressed their turning away from sin and turning to God seeking forgiveness for their sin.
Jesus baptism would be of the Holy Spirit which would be a cleansing and purifying baptism.
John also taught that Jesus would come and separate people as the farmer separated his crop of wheat. Jesus would be the one to come and judge mankind. He would separate those who had value by following His teachings (wheat) from those who continued to be wicked and have no Kingdom value (chaff). And like the farmer who burned the worthless chaff, the people who continued in wickedness would burn in a never ending fire.
Jesus was also baptized by John and as He prayed the heavens opened up and the Holy Spirit in the form of a dove descended on Him. A voice from heaven then said (verse 22) "You are my dearly loved Son, and you bring me great joy."