Ephesians 4:5's one baptism is 1 Corinthians 12: 13's baptism into the Body by the Spirit, the moment one believes Christ died for their sins, Romans 5:1; Romans 6: 1-6.
And what that bapistism, is the following one WITH Him, that the faith of the operation of God performs the moment one trusts Christ...
Luke 12:50 But I have a baptism to be baptized with; and how am I straitened till it be accomplished!
That baptism is this one...
Matthew 26:39 And he went a little farther, and fell on his face, and prayed, saying, O my Father, if it be possible, let this cup pass from me: nevertheless not as I will, but as thou wilt.
In other words, the Cross the Believer is identified having died on WITH Him the moment he or she trusts that Christ died for his or her sins...
Colossians 2:10 And ye are complete in him, which is the head of all principality and power: 2:11 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: 2:12 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. 2:13 And you, being dead in your sins and the uncircumcision of your flesh, hath he quickened together with him, having forgiven you all trespasses; 2:14 Blotting out the handwriting of ordinances that was against us, which was contrary to us, and took it out of the way, nailing it to his cross; 2:15 And having spoiled principalities and powers, he made a shew of them openly, triumphing over them in it.
Paul then addresses the endless treadmill of the works for salvation that some will right away add to that baptism's complete work...
2:16 Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days: 2:17 Which are a shadow of things to come; but the body is of Christ. 2:18 Let no man beguile you of your reward in a voluntary humility and worshipping of angels, intruding into those things which he hath not seen, vainly puffed up by his fleshly mind, 2:19 And not holding the Head, from which all the body by joints and bands having nourishment ministered, and knit together, increaseth with the increase of God. 2:20 Wherefore if ye be dead with Christ from the rudiments of the world, why, as though living in the world, are ye subject to ordinances, 2:21 (Touch not; taste not; handle not; 2:22 Which all are to perish with the using ; after the commandments and doctrines of men? 2:23 Which things have indeed a shew of wisdom in will worship, and humility, and neglecting of the body; not in any honour to the satisfying of the flesh.
Again, Ephesians 4: 5's baptism is performed by the Spirit of God and is the principle of identifying (the person who has trusted that Christ died for one's sins) WITH CHRIST.
Romans 7:4 Wherefore, my brethren, ye also are become dead to the law by the body of Christ; that ye should be married to another, even to him who is raised from the dead, that we should bring forth fruit unto God. 7:5 For when we were in the flesh, the motions of sins, which were by the law, did work in our members to bring forth fruit unto death. 7:6 But now we are delivered from the law, that being dead wherein we were held; that we should serve in newness of spirit, and not in the oldness of the letter.
The Spirit identifies the Believer as having died to the Law by the body of Christ the Believer is described in Romans 6 as having been identified with that the Believer should, that is, that the result of that identification WITH Christ in HIS, Death, Burial, and Resurrection WITH Him, should be that the Believer is now married or made one WITH Christ, NOT ONLY in in HIS, Death, Burial, and Resurrection, but in HIS newness of life IN Him, which is the Believer's enablement to serve.
Romans 5:19 For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous. 5:20 Moreover the law entered, that the offence might abound. But where sin abounded, grace did much more abound: 5:21 That as sin hath reigned unto death, even so might grace reign through righteousness unto eternal life by Jesus Christ our Lord.
Romans 6:1 What shall we say then? Shall we continue in sin, that grace may abound? 6:2 God forbid. How shall we, that are dead to sin, live any longer therein? 6:3 Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death? 6:4 Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life. 6:5 For if we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection: 6:6 Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin. 6:7 For he that is dead is freed from sin. 6:8 Now if we be dead with Christ, we believe that we shall also live with him: 6:9 Knowing that Christ being raised from the dead dieth no more; death hath no more dominion over him. 6:10 For in that he died, he died unto sin once: but in that he liveth, he liveth unto God. 6:11 Likewise reckon ye also yourselves to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord.
And so on.
In contrast this Baptism by the Spirit INTO the Body of Christ, the Baptism that Peter is describing in the following, is a water baptism...
1 Peter 3:20 Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water. 3:21 The like figure whereunto even baptism doth also now save us (not the putting away of the filth of the flesh, but the answer of a good conscience toward God,) by the resurrection of Jesus Christ:
That is this, an Israelite water purification ritual which was phased out by the time of Ephesians 4, which was written after Paul's final pronouncement against the Nation Israel, in Acts 28...
John 3:22 After these things came Jesus and his disciples into the land of Judaea; and there he tarried with them, and baptized. 3:23 And John also was baptizing in Aenon near to Salim, because there was much water there: and they came, and were baptized. 3:24 For John was not yet cast into prison. 3:25 Then there arose a question between some of John's disciples and the Jews about purifying. 3:26 And they came unto John, and said unto him, Rabbi, he that was with thee beyond Jordan, to whom thou barest witness, behold, the same baptizeth, and all men come to him.
By the way, I hold an Acts 9 Dispensationalism aka Mid-Acts, see my OP, and also, post # 30, linked below.
Are There Really Two Gospels?
You might also find my post after this one sheds some added light on our Two Gospels position...
Acts 17: 11, 12