Ephesians 4:5
one Lord, one faith, one baptism.
Some may claim that since water baptism is to occur that would make 2 baptisms. However, the one baptism in which every Christian has been placed into the body of Christ refers to the baptism with the Holy Spirit.
If it is insisted upon that "one baptism" means there is ONLY one baptism left (water baptism) then that encounters an insurmountable problem, because the apostle John (years after Ephesians was written) still had a "baptism" to undergo (his death).
Mark 10:35-39
(35) James and John, the two sons of Zebedee, came up to Jesus, saying, “Teacher, we want You to do for us whatever we ask of You.”
(36) And He said to them, “What do you want Me to do for you?”
(37) They said to Him, “Grant that we may sit, one on Your right and one on Your left, in Your glory.”
(38) But Jesus said to them, “You do not know what you are asking. Are you able to drink the cup that I drink, or to be baptized with the baptism with which I am baptized?”
(39) They said to Him, “We are able.” And Jesus said to them, The cup that I drink you shall drink; and you shall be baptized with the baptism with which I am baptized.
New International Dictionary of New Testament Theology (NIDNTT): the cup is understood as the cup of suffering, the baptism as the baptism of death (Matt. 20:22; Mk. 10:38) (2:276, Hunger, G. Braumann).
The Gentiles were baptized with the Holy Spirit (Acts 11:16).
Acts 11:16
And I remembered the word of the Lord, how He used to say, ‘John baptized with water, but you will be baptized with the Holy Spirit.’
The Gentiles being baptized with the Holy Spirit (Acts 11:16) = The Gentiles being given the Holy Spirit (Acts 15:8)
This also refers to: The Gentiles being sealed with the Holy Spirit, which Ephesians 1:13 applies to all who are Christians. Thus, the "one baptism" (Ephesians 4:5) refers to being baptized witrh the Holy Spirit (cf. Ephesians 1:13).
Acts 15:7-8
(7) After there had been much debate, Peter stood up and said to them, Brethren, you know that in the early days God made a choice among you, that by my mouth the Gentiles would hear the word of the gospel and believe.
(8) And God, who knows the heart, testified to them giving them the Holy Spirit, just as He also did to us.
Ephesians 1:13
In him you also, when you heard the word of trtuh, the gospel of your salvation, and believed in him, were sealed with the promised Spirit.