Ephesians 4: 4, 5. "There is one body, and one Spirit, even as you are called in one hope of your calling". "ONE Lord, ONE faith, ONE baptism". The theme is of salvation, and the baptism is the baptism into the body of Christ, by the Holy Spirit. This is the saving baptism spoken of in Mark 16: 16, where just belief, without repentance and the baptism by the Spirit into the body of believers one is not saved. See Romans 8: 8, 9. which reveals that we are part of the body, evidenced by the indwelling Spirit.