A caller to the Basic Gospel broadcast argued that both water baptism and spiritual baptism were necessary for salvation. With this as his premise, he wondered why Paul used the phrase "one baptism" in Ephesians 4:4-6.

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.

Paul does have it right. There is only one baptism that saves. To the surprise of many, it is not water baptism. 

The Holy Spirit is the one who regenerates us and baptizes us into the body of Christ. Paul explained in Romans 6. There he said that we have been baptized, or placed, into Christ’s death, burial and resurrection. We were also "baptized into one body" and made members of Christ's body. Though we are many, Paul wrote, we form one body.

There are many people in this world who have been baptized in water. Simon the Sorcerer was one such person. Luke recorded in Acts 8 that Simon believed and was baptized. Did this act save him? Of course not.

He went through the motions in hopes to add to his mystic and his financial empire. And when he offered money to have the same power of the Apostles, Peter called him out saying that he was still captive to sin. He was immersed in water, but that act didn't change his heart. 

Here is an important truth to understand. Water baptism is not the sign that a person is saved. It is a wonderful celebration of the work of the Holy Spirit in a person's life to save that person and add them to the body of Christ. And it is wonderful for other believers to witness. But water baptism is not the baptism that saves.

Spiritual baptism saves. Which means, everyone who has been baptized by the Spirit of God into Christ is saved.