There is debate concerning whether the phrase, “once saved, always saved,” is biblical. The phrase is biblical, but we need to understand exactly what it means
It is biblical that once a person is truly saved, that person is always saved. Simply professing faith in Jesus does not save a person.
True salvation is the result of regeneration by God, which inevitably results in both a profession of faith in Jesus, as well as a new desire to live for and obey God.
Jesus answered him, “Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God.”John 3:3
Someone who has been regenerated by God, and thus has repented and believed in Jesus, is saved and always will be saved. This is because regeneration is a one-time act by God that cannot be undone.
If a person falls away from faith, then that means the person was never truly regenerated or saved in the first place.
They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us.1 John 2:19
The reason why “once saved, always saved” is true is because salvation is ultimately from, and dependent upon, God, and not ourselves. Because it is God who saves a sinner, if God has chosen to save a person, there is absolutely nothing that person can do to lose that salvation.
Everyone whom God has chosen to save will be regenerated, which will also always result ini lifelong repentance and faith. God is not forcing anyone to obey Him because He changes their entire nature in such a way that they want to repent and believe in Jesus.