The phrase “one baptism” in Ephesians 4:5 refers to the shared, full commitment of all believers to Christ, symbolized by both Spirit and water baptism. This shared experience unites believers into one body, forming the basis for Christian unity.
In Ephesians 4, Paul is telling the believers that they are to live in unity with one another (Ephesians 4:1–3). The grounding for that unity is found in our relationship with the Spirit, Jesus, and the Father. In God, we are “one body” (the church) and have “one hope” (salvation), “one faith” (same confession), and “one baptism.”
The type of “baptism” is not specified. However, while Paul could very well be referencing either the Spirit or water baptism, the early church did not separate these as distinctly as we do. To be a believer means one was baptized by the Spirit at conversion (John 1:33). Likewise, members of the early church were baptized in water almost immediately after being saved (Acts 2:38; 8:34–39). Therefore, all Spirit-baptized believers were also water-baptized believers. Rather than trying to determine a specific type of baptism, it is better to understand Paul talking about the net result of baptism: the believer’s full immersion into the body of Christ. Thus, “one baptism” simply means that all believers are fully committed to Christ, joined to the body through the internal regeneration of the Spirit and known as members of the body by their public baptism.
As believers, we share the same fundamental beliefs and relationships with God as all other believers. That is a relationship that comes by the gift of the Spirit to us and evidenced by our public baptism. A believer cannot be partially baptized any more than he can be partly a Christian. Therefore, Paul says that our shared experience of baptism, among other things, allows us to be unified in peace (Ephesians 4:3). We obtain that unity by walking in a manner worthy of our calling as brothers and sisters in Christ (Ephesians 4:1). That means that we gently and humbly show tolerance for other believers out of love and patience (Ephesians 4:2).
Until Jesus returns, we will always find ways to disagree. This is, in part, because we are not yet perfect and misunderstand Scripture, coming to different conclusions about less important aspects. Therefore, there will always be tension among believers throughout the world until Jesus returns. However, how we treat one another despite the differences is a witness to the world just how great our God is.
Jesus said, “By this all men will know that you are My disciples, if you have love for one another” (John 13:35). It is our love—our loving of one another despite our disagreements—that stands out in a world that is constantly fracturing and fighting based on even the smallest difference between people.