To quote Paul Washer, "The greatest heresy in the American, Evangelical, Protestant church is that if you pray and ask Jesus to come into your heart he will definitely come in. You will not find that in any place in scripture."
http://youtube.com/watch?v=AYsClDclvf0
It's true. You cannot find a single passage saying, "Pray and ask Jesus into your heart." Jesus preached repentance, a turning away from sin, a desire not to be like the world, a desire to glorify God in everything you do; Jesus never preached that saying a prayer and asking him into your heart makes you a Christian. Instead, Jesus said,
Not everyone who says to me, "Lord, Lord," will enter the kingdom of heaven, but the one who does the will of my Father who is in heaven. On that day many will say to me, "Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?" And then will I declare to them, "I never knew you; depart from me, you workers of lawlessness" (Matthew 7).
Whether or not you said a prayer one time has nothing to do with whether God created a new heart in you. Regeneration is a supernatural work of God where he takes dead hearts of stone and breathes new life into them, not a work of a prayer.
For God, who said, "Let light shine out of darkness," has shone in our hearts to give the light of the knowledge of the glory of God in the face of Jesus Christ (2 Corinthians 4:6).
And God has promised that anyone who he breathes new life into he will preserve to the end.
I am sure of this, that he who began a good work in you will bring it to completion at the day of Jesus Christ (Philippians 1:6).
I am sure that neither death nor life, nor angels nor rulers, nor things present nor things to come, nor powers, nor height nor depth, nor anything else in all creation, will be able to separate us from the love of God in Christ Jesus our Lord (Romans 8:38-39).