W. Austin Gardner
Baptism is the outward expression of an inward experience. As one of the two ordinances Christ left with His church, it is of great significance. For the believer it is a time of triumph. He takes his stand in an element that spells death to all that he is by natural birth—water; he is immersed to symbolize his burial; then he is raised by the power of another’s arm to stand on the resurrection side of death in newness of life. The act of baptism symbolizes in a graphic and memorable form the great thing that has already happened in his heart. By virtue of Calvary and his faith in Christ he has been identified with Christ in His death, His burial, His resurrection. He has passed from death unto life. Baptism proclaims that.
For the onlooker it is a time of testimony. The believer witnesses to the world that he is now to be identified with Christ, the One the world has rejected.
John Phillips, Exploring Acts: An Expository Commentary, The John Phillips Commentary Series (Kregel Publications; WORDsearch Corp., 2009), Ac 8:38.