QUESTION: According to Ephesians:4:5, there is "one baptism." What is that "one baptism"?
RESPONSE: Actually there are seven baptisms in Scripture: 1) the baptism of the children of Israel "unto Moses in the cloud and in the [Red] sea"; 2) "the baptism of John [the Baptist]...unto repentance" (Mt 21:25; Lk 3:3; Acts:19:3); 3) Christ's baptism by John in Jordan; 4) the baptism of believers "with the Holy Ghost and with fire" by Christ (Mt 3:11; Mk 1:8; Lk 3:16; Jn:1:33; Acts:1:5; 11:16); 5) the baptism of the Cross in which we all share (Mt 20:22; Lk 12:50; Gal:2:20); 6) the believers' baptism with Christ into His death, burial, and resurrection symbolized in believers' water baptism (Acts:8:12, 36-38; Rom:6:3, etc.); 7) and the baptism by the Holy Spirit into the body of Christ (1 Cor:12:13).
Then why does Paul say there is "one baptism"? Because each of the seven occurs only once. Furthermore, they all testify to the "one faith" (Eph:4:5,12).