And as they were going along the road, they came to some water, and the eunuch said, "See, here is water! What prevents me from being baptized?" Acts 8:36, ESV
There is great continuity throughout the New Testament concerning baptism. Jesus said, "Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit..." Matthew 28:19, ESV. Taking a broad perspective of the New Testament, consider the following:
Baptism of repentance: This was the baptism of John the Baptist. It was a sign of those who were turning from sin and turning to the Lord. It was baptism by immersion. Jesus, as he was baptized by John, came up out of the water, the Spirit descended on him like a dove and a voice came from heaven saying, "this is my beloved Son, with whom I am well pleased." Not only did this serve to identify him as Christ the Lord, but Jesus modeled the act of baptism to the church.
Spirit Baptism: After Jesus died on the cross, he revealed himself to the disciples, ascended, sat at the right hand of the Father, and sent his promised Spirit to indwell the church. Today, he continues to draw believers to himself. The Spirit of Truth empowers each believer to carry out the work of God. Those who do not have the Holy Spirit do not have the Spirit of Christ. The Bible says, "...we were dead in our trespasses and sins, [Christ] made us alive..." Ephesians 2:5
Believers are in union with Christ in the Spirit. When a person places their faith in Jesus as Lord, that he died on the cross for their sin and rose physically from the dead, they become a child of God. Chosen by him, they are placed in union with Christ and marked with the seal of the Holy Spirit until the day of redemption. The new believer is spiritually immersed into the life of Christ. Just as the Holy Spirit empowered Christ to rise from his physical death, the believer is risen from the death of their sin to life in Christ. This is the baptism of the Spirit.
What about the filling of the Spirit? The apostle Paul encourages believers to be continually filled with the Spirit in his letter to the Ephesians. This ongoing work of the Spirit occurs as hearts submit to the will of God and believers walk with the mind focused on glorifying Christ. Believers securely rest in their union with Christ based on his work alone. Their relationship with God is not a continual struggle to win his favor. Christ finished the work of redemption once and for all on the cross. Their relationship with the Father is settled. Believers are encouraged to invite the Holy Spirit to empower their daily walk with him.
Water Baptism is the public profession of believers modeled in Scripture. Those who step forward for water baptism emphasize their identity with Christ and the church. Baptism is a time to emphasize the commonality of faith in Christ, over and above race, ethnicity, age, socio-economic, professional, or civil status. Through the public profession of water baptism, every tribe and tongue is brought together. This is the believer's profession of commitment to grow together in a Christ-centered community through relationships with others within the local body.
In Acts 8:36, when the eunuch said, "See here is water! What prevents me from being baptized!" The Spirit had drawn and enlightened him to the faith. His heart was bursting forth, united with Christ and empowered by the Spirit. His overwhelming desire was to profess his faith through water baptism.