Is re-baptism necessary for forgiveness of sins?
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Baptism, like marriage, is the sign of a union forever. It shouldn't have to be done repeatedly. When a husband and wife quarrel, do they get remarried afterwards in order to prove that they still love each other? Of course not. They forgive each other and get on with life. In Christ we have access to forgiveness of sins. That is the privilege upon which we depend when we fall away and wish to rededicate ourselves in God's service. We do not get re-baptized. We pray for forgiveness and as John says, in 1 John 1:9 If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness.
Where re-baptism might need to occur is if your first baptism was not valid. For a baptism to be valid it must be based on true understanding of the Gospel. See Acts 8:12. When your understanding of the Gospel fundamentally changes, then you will have to be rebaptized.
Here is a link to information which will be helpful to you:
Why Baptism REALLY Matters: www.thisisyourbible.com/library/show/178
I hope you have found this helpful.