Baptism and Wedding Rings
Baptism is like a wedding ring: they both symbolize transactions. A wedding ring symbolizes marriage, just as baptism symbolizes salvation. Wearing a wedding ring does not make you married any more than being baptized makes you saved. To extend the parallel, if a person, especially a woman, does not wear a wedding ring you can almost always assume that the person is not married.
So it was in New Testament times. If a person was not baptized, you could probably assume that he or she was not a believer. On this we must be clear: baptism is a symbol of salvation and only a symbol. But, like a wedding ring, it is such an effective symbol that it should never be taken for granted.
Michael P. Green, Illustrations for Biblical Preaching : Over 1500 Sermon Illustrations Arranged by Topic and Indexed Exhaustively, Revised edition of: The expositor's illustration file. (Grand Rapids: Baker Book House, 1989).