Although Jesus commanded water baptism as a part of the Great Commission (Matthew 28:18-20), the Bible does not say that baptism is necessary to receive eternal life. Eternal life is a free gift, one that we receive when we believe in Jesus (Ephesians 2:8-9; John 3:16). 

The Apostle Paul indicated that his primary purpose was to preach the Gospel, not to baptize people (1 Corinthians 1:17). He separated those two tasks, since baptism is an important step that comes after a person trusts in Jesus for eternal life. In 1 Corinthians 12:13, he says that believers are all "baptized by one Spirit into one body." Baptism is about being identified with a belief system or a person, and for the Christian the Holy Spirit is our identifying marker (John 1:33). Our water baptism is a public profession of something that occurred inside of us the moment we believed in Christ. The Spirit of God washed us clean (Titus 3:3-5), and baptism is a symbol that cleansing (1 Peter 3:21). 

Jesus commands us to baptize all Christians as a public statement of their faith in Him. However, there are clear instances in the Scripture of men and women who were not baptized yet who are clearly saved (e.g. the thief on the cross next to Jesus -- Luke 23:43). So baptism is not necessary to receive eternal life, but something we are called to do once we have believed in Christ.