The idea of "rededicating your life to Christ" is not named in the Bible, but that doesn't mean it's not useful. In fact, it can be an effective way to help people realize that Jesus' forgiveness is for all of us.
What is a Christian rededication? Should I rededicate my life to Christ?
There are two common scenarios in which people rededicate their lives to Christ. The most common is that of an older child or young adult who accepted Christ at a young age. After years of going to church and living through the influence of his parents' faith, he may realize his own faith is stagnant and underdeveloped. He may have never taken responsibility for his relationship with Jesus, or he may actually be living a sinful lifestyle. He comes to the realization that despite the fact he is a Christian, he wants a stronger Christian life. So he rededicates his life to Christ, taking a leap in maturity and restarting His Christian growth.
The second scenario is not strictly a rededication, but a realization. It involves someone who heard the gospel and thought she accepted Christ, but didn't understand the implications well enough to have a saving relationship with Jesus. She may have gone to church the whole time, even served, but at some point she comes to know and accept the true nature of salvation. If she doesn't realize that she was not a Christian before, she may call the transformation a rededication, even though it is technically a conversion.
Of course, it is not God's intent for any young Christian to fall into a sinful lifestyle. Romans 12:1-2 says that a believer is meant to reject sin and experience continual spiritual growth. Likewise, it's not God's plan for anyone to misunderstand the gospel, going through the motions of a Christian life for years, before really understanding saving grace.
But rededication as a concept is a powerful tool. It clearly demonstrates that God forgives. He forgives old Christians who sin, and new Christians who were deceived for years. It is a spiritual deep breath, wherein a believer can refocus her relationship with Christ. Like the Prodigal Son (Luke 15:11-32) and Peter in John 21, it shows that Jesus will always take us back.
A note about apostasy (see the book of Jude): Apostasy is when someone believes he is a Christian and goes through the motions, but then completely and irrevocably rejects Christ. Many people today fear they are apostates. But since the defining characteristic of an apostate is complete rejection of Christ, you can only identify one once they have died, still in rebellion. If you are breathing and want to rededicate your life to Christ, by definition, you are not an apostate. Forget about the labels and just return to the Father's love.
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