I could be the star employee of a company I work at; my ID card gives me access to the building and I have passwords to all of the programs. I'm the model employee.
And then I start stealing stuff until I get caught; I'm shown the door. My ID card has been deactivated and my password disabled; I'm persona non grata at the firm, and I badmouth the establishment to no end.
ALL of my efforts prior to my turn to crime have gone to nothing, because Original Sin, which is always present and which must always be fought against, won out and I fell out of grace with the Creator and then, with my employer.
Is it possible I could turn back away from the bad and convince the company to hire me again? Sure. But without that, were my name to come up in a reference check to that company, how could they possibly have anything positive to say about me? (By what right could I walk around telling people that this company which I badmouth will take me back, even as I badmouth it; no apology is necessary.)
It doesn't pass the logic test.
Okay. This won't be short and sweet. I warned you. :o)
Becoming/being a genuine born-again believer in Jesus Christ is more than simply being baptized Christian as a baby and being raised in a church or going forward during an invitation at a church service, it's more than hearing the gospel and it's even more than claiming to be a Christian. It's more than going to "church", giving money, obeying the commandments, being "good" or even telling others about Jesus. In fact, people can do all those things and still at the end hear the words from Jesus, "Depart from me I never knew you.".
So what IS becoming/being a genuine born-again Christian? It is a heart decision. It is a conscious act of faith:
That is what separates a genuine believer from a pseudo one. God certainly sees the heart - none of us can - and knows if our faith is genuine. He promises:
We are repeatedly told in Scripture that salvation is the gift of God. That it is by grace we are saved through faith and not by works lest anyone should boast. Those who come to God in faith are not turned away - we WILL be saved. We are born-again into the family of God. We are no longer just His creation, we are His children. We are indwelt and sealed by the Holy Spirit as a promise until the day of redemption. Will God break His promise? NO. Does God lie? NO.
We weren't justified by our works but by faith. We don't stay justified by our works either. We are being conformed into the image of Christ, God is at work in us:
So now with all that said, what do we say about those who said they were Christians but then abandoned their faith? I can think of a few things. A genuine believer WILL be transformed by the Holy Spirit. There is a process of sanctification - we are a work of God. But it won't work if there is no genuine faith.
But can a genuine believer become discouraged, have a weak faith, have doubts? Yes, we ARE human and we still have an old nature that wars against the new spirit nature (see Paul's issues in Romans 7). Even the disciples of Jesus who walked and lived with Him for three years had doubts after the crucifixion. If we're honest, we ALL go through periods of doubt because we are walking by faith and not by sight. Look at the Israelites and how easily they abandoned God as they went through hardships in the wilderness even though they saw many, many miraculous things God did for them. Yet, some of them retained their faith through it all. I believe that if a person is a true child of God, He is ready to deliver them through those times of doubt. Some people can go years in the valley of doubt but they WILL persevere. They cannot deny what has taken place within their hearts. We have to be reminded about the many times God has worked in our lives through answered prayer, providing our needs, giving us peace, surrounding us with His presence and promising to never leave us or forsake us. The Israelites were told to set up monuments where God had worked miraculously for them as reminders. We need those kinds of reminders, too. It's sometimes easy to forget all that God has done for us when we are faced with a new trial or time of testing.
I don't know what is in a person's heart that would cause him to publicly renounce his faith in Christ. Maybe it wasn't a real faith. Maybe he's going through a valley and will regain his faith one day. Only God knows but we can certainly pray for him. He may one day return to his faith and we know God will be faithful to forgive him and cleanse him from all unrighteousness. Maybe he will finally come to a genuine faith for the first time.
I have a family member who accepted Jesus Christ as Savior and had a strong faith for years. Then he went through some terrible times and renounced Christ and claimed to be an atheist. But now he has returned to his faith after many years in the valley of despair. Did he lose his salvation during that time of doubt or does God's mercy and grace cover such times? Whatever doubts a believer might be going through, there remains a born-again soul who God says He would never cast out or lose one. Won't the doubters faith be strengthened once he is in God's presence? You asked would God send someone who was once a believer to heaven who doesn't believe in it anymore? Well he will believe it when he sees it! Jesus told His disciples, “Have you believed because you have seen me? Blessed are those who have not seen and yet have believed.”. (John 20:29). Heaven is real as is God and Jesus Christ. The Christian faith is real and true. God will not disown His child if he is going through a period of doubt. His love is so much bigger than that. Thank God for His mercy and grace that not only overcomes sin it overcomes our lack of faith and won't we be eternally grateful that we didn't foolishly lose the gift of God because of our doubts! I hope this answers your question.
In your example of the employee stealing from their employer, let me give this response: if your boss found out about your crimes and brought you into his office and said, "Captain Walker, I know you have been stealing from me. You're fired but I'm not going to have you arrested and prosecuted. You are free to go but you can never work here again and no one will give you a reference." I think that's what we'd call MERCY - you didn't get what you deserved. I'd think you'd be relieved.
But if that same boss instead said, "Captain Walker, I know you have stolen from me and done wrong, but I will not have you arrested and prosecuted. Instead, I forgive you and I am promoting you and giving you a raise." That's what we would call GRACE - you are getting what you don't deserve. I think you'd be overwhelmed with gratitude!
Because of God's mercy we are not consumed, we are not going to hell for our sins because Jesus Christ died for our sins and we receive that mercy through faith. Because of God's grace we not only are forgiven of our sins we are adopted into His family and made heirs and joint heirs with Christ and will spend eternity in heaven with God. How great is God's love! When we ponder that beautiful truth, how can we be anything but grateful and desire to live a life that honors that great love, mercy and grace.