Luke/Acts records two conjoined accounts of baptism. The Gospel according to Luke retells the ministry of John the Baptizer. The Acts of the Apostles recounts among others, the ministry of Peter to the household of Cornelius. The words prophetic in John and fulfilled in the mind of Peter form the nexus of these baptisms. I [John] baptize you with water...He [Jesus] will baptize you with the Holy Spirit and fire.
I [John] baptize you with water...
John presented his baptism as one of repentance for the forgiveness of sins (Luke 3.3.) Forgiveness was the result of the prerequisite change of mind. Josh held participants in his baptism to a high expectation of "fruit in keeping with repentance." (3.8.) he refused to baptize some and admitted others. When confronted with his expectation, some in the crowd asked for clarification. They needed assistance in understanding what repentance looked like.
John gave three examples of "fruit in keeping with repentance." First, the truly repentant are concerned for the welfare of others. They share when they have something to wear or eat (verse 11.) Second, The truly repentant don't take advantage of others (verses 12-13.) They recognize the rights of both themselves and others. Lastly, the truly repentant cultivate contentment with what they have and don't take by force what belongs to others (verse 14.) Redirected minds redirect actions.
John's baptism of repentance for the forgiveness of sins emphasized the relation of the individual to the Covenant God offered rather than that of the people of God to that same Covenant. This call literally "laid the ax to the root of the tree" of dispassionate, disconnected relating to YHWH who passionately performed His Covenant duties. Claiming to belong, by natural birth alone, to the People of God did not suffice. "God can raise up children for Abraham" from inanimate, dispassionate objects like the river rocks lying along the Jordan river banks. Every individual needed to choose for themselves.
At least a few, and perhaps many, found offense in this call to personal responsibility rather than national identity. These had disconnected personal participation in the Covenant from the benefits of belonging to the Covenant People. Then, as now, God's grace invites the faithful to partner with Him in the work in them and in the world (2 Corinthians 6.1.) Personal responsibility, not group identity, produces "fruit in keeping with repentance."
But John defines repentance in terms of interactions with our fellowman. Answering the call for individual response to God's offer of Covenant finds expression in social responsibility. While no one can count on family (Abraham's children) or religious affiliation (Judaism) or geographic location (Judah, Jerusalem: capitals) for salvation, we express our saving relationship through these.
John's Baptism with Water physically demonstrated an inner change. The participant left the baptism changed -- dry became wet. One does not have to read creatively to imagine that this act could have been a dunking. As the participant was symbolically buried in the river, the crowd witnessed the end of their old way of believing and behaving. As they rose, the crowd saw a resurrection to their new way of believing and behaving.
Jesus affirms this baptism of repentance by participating in it (verse 21.) His presence sanctioned this baptism. He later commissions His disciples to continue this repentance-leading-to-forgiveness baptism (John 3.26, John 4.1-2.) His practice continued the baptism begun by John. Our Lord's final words to them were to "go...baptizing" (Matthew 28.19-20.) These facts bond the John's Baptism with Water with Jesus' Baptism with the Holy Spirit.