One baptism? Yes, one, according to Ephesians 4:5 quoted above. 

Which baptism is the one, then; since many are mentioned in scripture? I believe we find the answer in Acts 19:1-7. Here, we are told that while in Ephesus Paul baptized (immersed in water) 12 men in the name of Jesus. 

Paul did exactly what Peter taught thousands to do in Acts 2:38. He was doing the same thing Philip did in Samaria (Acts 8:12) and with the Ethiopian eunuch (Acts 8:35-38). Each of these men, Peter, Philip and Paul simply taught the death, burial and resurrection of Jesus and then baptized those who believed in the name of Jesus, that is, by his authority. 

It is of interest to me that Paul baptized the men in Ephesus in the name of Jesus in spite of the fact that they had already been baptized under the baptism of John. Obviously then, the one baptism was not the baptism of John. 

The men at Ephesus had also not been baptized with the Holy Spirit, as they confessed to Paul that they had not heard whether there was a Holy Spirit. After they had been baptized in Jesus name, Paul laid his hands on them so that they would receive the Holy Spirit. The one baptism then, is not Holy Spirit baptism. 

Baptism in Jesus’ name is the same one Ananias had encouraged Paul to receive, according to Acts 9:17-18 & 22:12-16. 

Baptism in the name of Jesus was what Peter and Philip preached. Ananias was sent by Jesus himself to baptize Paul in his name. Paul baptized the Ephesians in Jesus’ name. These men were simply fulfilling what Jesus himself had taught in Matthew 28:18-20 when he said: 

“All authority has been given to Me in heaven and on earth. Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit, teaching them to observe all that I commanded you; and lo, I am with you always, even to the end of the age.” 

The one baptism of Ephesians 4:5 is the one Jesus gave us to the end of the age.