Question: "What does it mean that there is only one baptism (Ephesians 4:5)?"
Answer: Ephesians 4:4-6 says, "There is one body and one Spirit, just as you were called to one hope when you were called; one Lord, one faith, one baptism; one God and Father of all, who is over all and through all and in all." Since there are different "baptisms" referred to in the New Testament, it can be a bit confusing when we read about "one baptism." The word baptize always means “to submerge or immerse.” So, when baptism is discussed, it involves a person being totally submerged into something else. Baptism implies being "all in." It also implies that a change has taken place. Baptized people are changed people.
Generally speaking, there are two types of baptism: a physical (water) baptism and a spiritual baptism. One is literal, done in water; the other is figurative, accomplished in the Spirit.
Water baptism was commanded by Jesus for all of His followers (Acts 1:8). Colossians 2:12 says, "Having been buried with him in baptism, in which you were also raised with him through faith in the powerful working of God, who raised him from the dead." Being baptized with water does not save us; faith in the finished work of Christ saves us (Ephesians 2:8-9; Romans 10:9). But water baptism is an outward indication of an inward change. It is a wonderful picture of Christ’s death, burial, and resurrection. Being immersed in water symbolizes the cleansing of our hearts and the washing away of our sin by the blood of Jesus (Acts 2:38). Through water baptism, believers publicly proclaim their testimony that they have been born again by the grace of God.
Romans 6:3 speaks of a spiritual baptism: "Or don't you know that all of us who were baptized into Christ Jesus were baptized into his death?" This spiritual baptism “into Christ” is performed by the Holy Spirit the moment a repentant sinner accepts the gift of salvation and is born again (John 3:5; Ephesians 2:18; 1 Corinthians 12:13; Acts 8:12). We respond to the Holy Spirit's drawing and are born into God's family (John 6:44; 1 Corinthians 6:19). By this "baptism," we are identified with the death and resurrection of Jesus; from then on, we consider ourselves "crucified with Christ" (Galatians 2:20). We choose to lose ourselves and be immersed in Him (Matthew 16:24), and the Holy Spirit makes that happen.
The baptism of the Holy Spirit was promised by John the Baptist, who said that Jesus “will baptize you with the Holy Spirit and with fire" (Luke 3:16). No one understood what John meant until after Jesus had ascended back into heaven (Acts 1:9). Jesus had promised the disciples that He would send "the Comforter" (John 14:26; 15:26; Luke 24:49). His followers were to wait in Jerusalem until the "promise from the Father" came (Acts 1:4). That promise came in Acts 2. The Holy Spirit was poured out on the disciples, and they were never the same again. They were bold in their witness, empowered to perform miracles, willing to endure persecution, and all but one died a martyr's death. The church had begun. Throughout the book of Acts, that baptism by the Holy Spirit was repeated as people came to know Jesus, both Jew and Gentile, and served to unify the church as the Jewish believers realized that the Holy Spirit was poured out on their Gentile brothers as well.
There are some differences of opinion among believers concerning the baptism of the Spirit. Some Christians believe Holy Spirit baptism is identical to being baptized into Christ and that it occurs at the moment of salvation, even if the believer is unconscious of it. Other Christians believe Holy Spirit baptism is to be equated with the filling of the Spirit and that often occurs after salvation—years later, perhaps—as the believer opens himself up to the Spirit’s control. Some believe that the baptism of the Spirit is always accompanied by signs (such as speaking in tongues), and others believe that such signs are unnecessary.
When Paul wrote to the Ephesian believers about "one baptism," he was reminding them that, regardless of their background or nationality, they all served the same Lord, shared the same faith, and had experienced the same baptism. He could be referring to water baptism; i.e., all believers have the same testimony of salvation and have been baptized in the name of the Father, Son, and Holy Spirit. Or he could be referring to Spirit baptism; i.e., all believers have been placed into the Body of Christ through the Spirit’s power. Either way, the emphasis is on unity among Christians. Verse 3 says, "Make every effort to keep the unity of the Spirit through the bond of peace." The Holy Spirit works to unify believers and provides assurance that they are children of God (Romans 8:16; Ephesians 1:13-14). By reminding the church that they all had a similar testimony and that they were all partakers in the same Holy Spirit, Paul encouraged them to work together for the cause of Christ so that the message of redemption would continue to spread throughout the world (Matthew 28:19).