It is necessary for Christians to believe in Jesus in order to be saved, and yet they do not earn their own salvation by their belief. Their salvation, and spiritual ressurrection, is brought about completely by the power of God; it is applied when the human subject believes in that power:
having been buried with him in baptism, in which you were also raised with him through your faith in the working of God, who raised him from the dead. (Colossians 2:12 NIV)
True Christians are spiritually buried and resurrected with Christ. This happens in baptism (immersion), not the baptism (immersion) of water, but the baptism of the Holy Spirit. This is done through our “faith in the working of God.” The Apostle Peter explains that the baptism that saves us is the spiritual baptism that happens the moment we believe, not the actual performance of the water ritual:
and this water symbolizes baptism that now saves you also—not the removal of dirt from the body but the pledge of a clear conscience toward God. It saves you by the resurrection of Jesus Christ, 22 who has gone into heaven and is at God’s right hand—with angels, authorities and powers in submission to him. (I Peter 3:21-22)
This spiritual baptism, which is symbolized by our water baptism, occurs when we have “a clear conscience toward God.” This happens when we repent of our sins and believe in Jesus Christ and the power of God to save our souls just as He raised Jesus from the dead. Physical baptism does not save us, but it is very important nonetheless, because it is an expression or “pledge” of our changed heart. Water baptism is not a suggestion for Christians: it is a commandment.
I cannot fathom how a person could become a true Christians and flat out refuse to be baptized. Baptism is a natural expression of joy and obedience to God that we are supposed to have done once we realize that God has saved us. Some people teach that you can only become a true Christian after you have been physically baptized, but we see an example in the Bible of God giving the Holy Spirit to new Christians at the moment of belief before they are baptized (Acts 10:47-48 NIV). Therefore our salvation is secure from the moment God has allowed us to believe. Those who believe will be saved even if they die before they have an opportunity to express their faith through water baptism.