Question: "I have sinned. Do I need to be baptized again?"
Answer: The question of whether a person who has sinned should be baptized again is a fairly common one. First, it is important that we understand what baptism is. Baptism does not save us or wash away our sins. Baptism is simply an illustration of what has occurred in a believer's life when he/she believes in Jesus Christ. Baptism illustrates a believer's union with Christ in His death, burial, and resurrection. Romans 6:3-4 teaches us, "Or don't you know that all of us who were baptized into Christ Jesus were baptized into his death? We were therefore buried with him through baptism into death in order that, just as Christ was raised from the dead through the glory of the Father, we too may live a new life." The action of going under the water pictures being buried with Christ. The action of coming out of the water illustrates Christ's resurrection and our identification with Him as we are raised to "walk in newness of life" (Romans 6:4 KJV).
Baptism is important because it is a step of obedience—a public declaration of faith in Christ and commitment to Him, and identification with Christ's death, burial, and resurrection. If we know Jesus Christ as Savior and understand what baptism signifies when we are baptized, then we do not need to be baptized again. If we did not know Jesus as Savior when we were baptized, then we do need to be baptized again. If we knew Jesus as Savior but did not truly understand what baptism signifies, then perhaps we do need to be rebaptized. But this is a matter of conscience between the believer and God.
It's also important to understand that believers will continue to sin, although sin should have less and less of a hold on us as we mature in Christ, and the incidence of sinning should continue to lessen throughout our lives. When we do sin, we should confess it to God, asking Him to forgive us and to restore our intimate fellowship with Him. We have the promise that "he is faithful and just and will forgive us our sins and purify us from all unrighteousness" (1 John 1:9). Nowhere does the Bible say we must be rebaptized to be forgiven.