The debate on the eternal security of a Christian and the possibility of a Christian losing his or her salvation has never been conclusive for clear Scriptural references have been put forth in support of both position. A better re-phrased question to ask is : How is a Christian justified by faith ?
I believe the crux lies in repentance. Repentance and saving faith are two sides of the same coin. Though the Greek word for repentance, metanoia, means a change of mind, the Bible is very clear that repentance involves fruit or deed in line with it. Luke 3:8-14; Acts 26:20; Acts 3:19. In Acts 2:38, we are told to repent and be baptized in the name of our Lord Jesus for the forgiveness of sins, and then to receive the Holy Spirit.
So a true believer changes his mind from rejection of Jesus Christ into full acceptance of Him as his personal Savior and Lord. And a true change of mind will result in a changed life. 2 Cor 5:17; Gal 5:19-23; James 2:14-26.
But even this repentance is the work of God in our lives starting from disbelief changing into knowledge with God pulling us and then to wisdom when He opens our eyes. Eventually convicting us to make that decisive response of following Jesus as our Savior and Lord.
A Christian is not pre-disposed to sin anymore. Though he sins occasionally due to temporary weakness or strong temptation, his life is overall walking right with God. A good example is King David. 1 Kings 15:5 clearly explained that : For David had done what was right in the eyes of the Lord and had not failed to keep any of the Lord’s commands all the days of his life—except in the case of Uriah the Hittite. When his sin was pointed out, his immediate response was to realize his sin, feel godly sorrow and seek God’s forgiveness. That is the overall life of a regenerate believer. (For full clarity, read the whole epistle of 1 John.)
An unregenerate believer or a professing believer is one who is overall indifferent to sin. You will notice his chronic sinful behavior. You will notice most, if not all, his decisions are worldly and not according to the instruction in the Bible.
(1 Cor 6:9-10. Or do you not know that wrongdoers will not inherit the kingdom of God? Do not be deceived: Neither the sexually immoral nor idolaters nor adulterers nor men who have sex with men nor thieves nor the greedy nor drunkards nor slanderers nor swindlers will inherit the kingdom of God.)
Another good example is Lot. Though he lived in Sodom and seemed to compromise at times, and even hesitated to leave Sodom before it was destroyed, the Bible told us in 2 Pet 2:8 that he was tormented by the lawless deeds he saw in the city.
A regenerate believer will be distressed by sin. That is why the hyper grace gospel is so dangerous. From the onset, the false gospel downplays sin altogether and actually trivializes the tremendous perfect sacrifice of Christ our Lord for the atonement of sins. Heb 10:26-29.
A true believer is a new creation and he will love and obey the commandments of God as the fruit in line with repentance.
Gal 6:15 For neither circumcision counts for anything, nor uncircumcision, but a new creation.
1 Cor 7:19. For neither circumcision counts for anything nor uncircumcision, but keeping the commandments of God.
There is no question over the clear distinction between regenerate and unregenerate believers. That leave only this last doubt. Can a regenerate believer ever walk out of faith himself by returning back to a life of sin again ?
Let’s look at the parable of the prodigal son for a possible insight. The younger son came to his senses acknowledging his sin and repented. He went back with so much gratitude for the unconditional love of his father that he was prepared to start over without the privileges of son-ship. Would he betray the trust of his father a second time and return back to the far country of reckless living again if his repentance was unequivocal ?