The use of water in a symbolic manner to indicate spiritual washing is widespread in most religions. At the time of Jesus new converts to Judaism were immersed and the males were circumcised.  The word baptism was also used of the ritual washings carried out by the Pharisees.

...and when they come from the market place, they do not eat unless they purify (baptize) themselves; and there are many other traditions which they observe, the washing of cups and pots and vessels of bronze.  (Mark 7:4)

The baptism carried out by John the Baptist was unusual in that he was calling upon Jews to repent of their sins and to show this by baptism in the Jordan. In effect, he was stating that they needed to be converted to true faith. Jesus too was baptized by John, not for the forgiveness of his sins but as a mark of his dedication to God’s will. It was reported that Jesus was soon baptizing more followers than John.

Now when the Lord knew that the Pharisees had heard that Jesus was  making and baptizing more disciples than John (although Jesus himself did not baptize, but only his disciples)....                                    (John 4:1-2)

Instructions to baptize were specifically given by the risen Jesus in his commission to the disciples (Matthew 28:18-20).

In the early ecclesias, therefore, baptism in water was the symbolic act by which those who had come to believe, Jew or Gentile, became members of  the Christian Community. In Paul’s teaching (1 Corinthians 12:12-13, Galatians 3:26-29, Romans 6:1-4, Colossians 2:9-13) the meaning of this act is repeatedly drawn out. Those who have been baptized have “put on Christ”, they have died to an old way of life, and have been raised in Christ to a new manner of life where all sins have been forgiven and dividing barriers of class, race or gender are eliminated. In line with what we have seen already of the attitude of Jesus, it is not the ritual act in itself but what it represents before God in the heart of the believer which gives Christian baptism its meaning and importance.

Baptism ... now saves you, not as a removal of dirt from the body but as an appeal to God for a clear conscience, through the resurrection of Jesus Christ....                                                                                  (1 Peter 3:21)

Should we therefore stress the spiritual change but eliminate the physical act of baptism as simply a practice related to first century conditions? There are three reasons why we should not:

(1) Jesus himself was baptized, endorsing the practice by example (Matthew 3:15).

(2) During his ministry Jesus encouraged his disciples to baptize and after the resurrection he commanded them to baptize all nations.

(3) This commandment was put into practice for all converts as can be seen repeatedly throughout the New Testament.

Baptism is therefore a practice specifically enjoined upon the ecclesia by Jesus himself.  It has a wealth of meaning in marking the change from an old way of life to a new, in marking the entry into the body of Christ in which we are all united together, and in holding out promise of eternal life in the future.