Why did he say there is "one baptism?" First, let's take a look at this one baptism.
Col. 2: 11-13
"In whom also ye are circumcised with the circumcision made without hands, in putting off the body of the sins of the flesh by the circumcision of Christ:
Buried with him in baptism, wherein also ye are risen with him through the faith of the operation of God, who hath raised him from the dead.
And you, being dead in your sins and the uncircumcision of your flesh, hath he quickened together with him, having forgiven you all trespasses;"
This is the "spiritual baptism" the "one baptism" that Paul is speaking of. It takes place the instant of hearing the Gospel and believing in His finished work. It's not water baptism, it is what Jeremiah spoke of "the circumcision made without hands," Paul called it "baptized into Christ" throughout His letters. It has nothing to do with water, it's your true salvation in the eyes of God.
So what about the other 2 baptisms found in the New Covenant, water baptism and the baptism of the Holy Spirit with evidence of speaking in other tongues? Paul said there is "one baptism."
Water baptism is not a baptism to itself, it is the reenactment of the "spiritual baptism," which is your true salvation. In the "spiritual baptism" you are baptized/immersed into Christ spiritually, symbolically done in water being immersed in Christ.
"Buried with Him in baptism" (spiritually), symbolically buried in the water. "Raised a new creature in Christ" symbolically being raised out of the water. So water baptism is a part of the "one baptism" spiritually performed by Christ. In other words, water baptism is symbolic of the one true spiritual baptism. Which means the only purpose of water baptism is to give a literal reflection of the one true spiritual baptism, which is also our literal confession of salvation.
The "baptism of the Holy Spirit with the evidence of speaking in other tongues" in spite of what the Pentecostals claim, is not a baptism! It is a "filling/receiving of the Holy Spirit" and is always referred to in that manner.
The only one who literally says, "baptized with the Holy Spirit" is John the Baptist. Before the Pentecostals came along claiming this is what John the Baptist spoke of, all the great theologians agreed that John the Baptist was speaking of "the one baptism" the spiritual baptism into Christ. In other words, the instant of our salvation. It is through faith in the finished work of Christ that the Holy Spirit, spiritually baptizes us into Christ, "the one baptism."
This is where all the confusion comes from with the Pentecostals "baptism of the Holy Spirit." They have given it a name that has already been given meaning!