This is in response to an adult convert to Christianity who wrote to ask about the history and significance of the Christian sacrament of baptism.
Baptism is an outward act that symbolizes the inward phenomenon of coming to and accepting Jesus Christ as real, as God incarnate, as the sacrificial means by which those who believe in him can be forever reconciled to God.
Baptism comes up in the New Testament quite a bit, and always at a critical time in the lives of those participating in it.
People who are newly saved are usually deeply moved to in some profound manner physically express that glorious transformation in their lives, to do something that shows and captures what's happened to them. Among its other significant meanings, baptism very often serves that wonderful purpose in the lives of those who are born again.
In essence, the symbolism of baptism is that, just as Christ died and was buried, so the baptized person is submerged (whether physically or symbolically) under water. Each is enveloped by the most earthly of elements; thus is the mortality of each emphasized.
And just as Christ rose again from beneath the earth, so the baptized person rises again from beneath the water.
Under the water is the believer’s old, dead, heavy, suffocating life. Out of the water, cleansed by the blood of Christ, is the believer’s new, fresh, purposeful life.
Like anything in the Bible that is plainly foundational to the Christian faith, over the course of 2,000 years baptism has been considered and practiced in light of a great variety of understandings and traditions. Today different schools of Christian belief hold varying ideas about who should and shouldn't be baptized--and when, and how. Some hold that only adults should be baptized; others believe in infant baptism. Some think you should get baptized as soon as you are saved; others that the act, being but an outward sign of sanctification, can be postponed. Some believe that the recently saved should wait to get baptized until he or she has sufficiently "grown in Christ." Many churches won't allow a person to belong to that church unless they are first baptized in that church.
And then there's the whole "dunk or sprinkle?" split on the physical act of baptism itself. Some Christian denominations practice whole-body immersion; some find a light sprinkling or touch of water to the head symbolically sufficient.
It’s a fair guess that all these sorts of distinctions mean a good deal more to man than they do to God. If you are wondering whether or not you personally should get baptized, I would recommend first finding the church that's right for you (see my How to Find the Right Church for You), and then following whatever they do in that regard.
No matter when or how you get baptized, know that you're in the best possible company. For Jesus himself began his ministry on earth by receiving a baptism from (whom else?) John the Baptist.
Read it and weep (with joy):
I baptize you with water for repentance. But after me will come one who is more powerful than I, whose sandals I am not fit to carry. He will baptize you with the Holy Spirit and with fire. Matthew 3:11
Then Jesus came from Galilee to the Jordan to be baptized by John. But John tried to deter him, saying, "I need to be baptized by you, and do you come to me?" Jesus replied, "Let it be so now; it is proper for us to do this to fulfill all righteousness." Then John consented. As soon as Jesus was baptized, he went up out of the water. At that moment heaven was opened, and he saw the Spirit of God descending like a dove and lighting on him. And a voice from heaven said, "This is my Son, whom I love; with him I am well pleased." Matthew 3:13-17
You know what has happened throughout Judea, beginning in Galilee after the baptism that John preached -- how God anointed Jesus of Nazareth with the Holy Spirit and power, and how he went around doing good and healing all who were under the power of the devil, because God was with him. Acts 10:37-38
... and this water symbolizes baptism that now saves you also—not the removal of dirt from the body but the pledge of a good conscience toward God. It saves you by the resurrection of Jesus Christ, ... 1 Peter 3:21
For we were all baptized by one Spirit into one body—whether Jews or Greeks, slave or free—and we were all given the one Spirit to drink. 1 Corinthians 12:13
And now what are you waiting for? Get up, be baptized and wash your sins away, calling on his name. Acts 22:16