In Acts 2:38 Peter is recorded as saying "Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost".  The gift of the Holy Ghost was to occur in conjunction with being baptized. 

In verse 39 Peter says, "For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call."  We see that the gift of the Holy Ghost was a promise.  In Ephesians 1:13-14 we see the same Holy Spirit of promise mentioned by Peter on the day of Pentecost.  We learn in Ephesians that the Holy Spirit of promise is the earnest or security deposit of our inheritance, given to those who believe, (Ephesians 1:13) and are obedient, as taught by Luke in: Acts 5:32, "And we are his witnesses of these things; and so is also the Holy Ghost, whom God hath given to them that obey him".

Paul further reinforces this in his letter to Titus where he says Jesus "saved us, by the washing of regeneration, and renewing of the Holy Ghost", (Titus 3:5).  To the Corinthians, Paul wrote, "but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of our God".

Baptism and the Holy Spirit are in conjunction with each other.  Baptism is described in scripture as a burial into the death of Jesus, (Romans 6:4), and a resurrection to walk in newness of life.  Newness of life is having a new life in place of the old, or, "born again".  Jesus taught Nicodemus that, "Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God", (John 3:5).  The rebirth of water and of the spirit is a simultaneous event that happens at one's baptism in water.  "Born of water and of the Spirit" is the re-birth Jesus says must happen in order to "see the kingdom of God", (John 3:3). 

In John's first epistle he wrote, "And there are three that bear witness in earth, the spirit, and the water, and the blood: and these three agree in one", (1 John 5:8).  So we see that the Spirit of God is a witness with the water and the blood of Christ.  Therefore it can only be that at our baptism in water, for the remission of our sin, (Acts 2:38, 22:16), we come into contact with the cleansing blood of Christ that "washes us from our sins", (Revelation 1:5), and God sends the "Spirit of his Son into your hearts, crying, Abba, Father" (Galatians 4:6).  All three elements come together in one to bear our witness in earth. 

This why when Peter wrote concerning baptism in 1 Peter 3:21, he said it "doth also now save us".

Series of studies on Baptism

Baptism Defined | New Testament Baptism | Which Baptism is it?  | Baptism of The Holy Spirit
The Gift of the Holy Spirit

Related Studies

The significance of Baptism  |  Is Baptism defined as work in scripture?   |  Examples of Christian Converts