If you’re not familiar with what is called “the sinner’s prayer,” it is a prayer that basically repeats the facts of the simple message of salvation. There are many variations of the prayer, but it usually sounds something like, “God, I know I’m a sinner and deserve to die for my sins; but I know that Christ died for my sins, and rose again. I believe this, and I ask You to save me.”
Now, that certainly sounds innocent enough, doesn’t it? I mean, it’s just praying the specifics of the gospel out loud (1 Cor. 15:3,4). What could be wrong with that?
The thing that makes praying the sinner’s prayer not a good idea is that God never asks us to pray the gospel, He only asks us to believe the gospel. So asking someone to pray the gospel is adding to God’s simple requirement for salvation. And, if you think it through, it is adding a work to the receiving of something God says is a gift (Rom. 6:23).
Perhaps you are thinking, “Prayer is a work?” Well, in writing to the Colossians, the Apostle Paul spoke well of a man named Epaphras, whom he described to them as “always laboring fervently for you in prayers” (Col. 4:12). If you can labor in prayer, then prayer must be a work. It may not be a very big work, but salvation today is by grace through faith alone (Eph. 2:8,9; Titus 3:5), and the moment you add even the slightest work to grace, it ceases to be grace (Rom. 11:6).
Another problem with asking a sinner to pray the sinner’s prayer is that years later he/she may wonder if they prayed it right, if they said the right words in the right way. And since the sinner’s prayer is not in the Bible, they cannot check to see if they prayed it right, which may cause them to question if they are truly saved.
Now if you prayed the sinner’s prayer when you were saved, you needn’t wonder if praying this prayer somehow negated the gospel you believed to be saved. Many Christian have prayed this prayer as well, but praying the gospel isn’t what saved us. Believing the gospel is what gave us eternal life. The moment you believed the gospel, you trusted Christ with your eternal destiny, a process Paul describes in Ephesians 1:13 with no mention of praying the sinner’s prayer. Speaking of Christ, he says,
“In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that Holy Spirit of promise.”
Paul says that after you believed the facts of the gospel, you were saved and sealed with the Spirit. God did not wait until you asked to saved by repeating the facts of the gospel in prayer. No, you were sealed for time and eternity from the very moment of belief. source