True Christians cannot lose their salvation. Scripture teaches that salvation is completely dependent upon God and not upon human choice. Although humans need to have faith to be saved, this faith itself is a gift from God.
Ephesians 1:3-4 teaches that God has chosen whom He will save before the creation of the world.
3 Blessed be the God and Father of our Lord Jesus Christ, who has blessed us in Christ with every spiritual blessing in the heavenly places, 4 even as he chose us in him before the foundation of the world, that we should be holy and blameless before him.Ephesians 1:3-4
Since a person’s salvation is determined before birth by God, nobody whom God has chosen to be saved can lose salvation. This does not mean God forces anyone to be saved, because God saves people by changing their entire nature in such a way that they want to repent and have faith in Jesus.
Romans 8:29-30 teaches that there is a “Golden Chain” from God “foreknowing” and “predestining” a person all the way until God “glorifies” that person. There is nothing that can break this chain.
29 For those whom he foreknew he also predestined to be conformed to the image of his Son, in order that he might be the firstborn among many brothers. 30 And those whom he predestined he also called, and those whom he called he also justified, and those whom he justified he also glorified.Romans 8:29-30
Likewise, Philippians 1:6 says this:
6 And 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.
There are professing Christians who fall away from the faith. 1 John 2:19 teaches that people who fall away from the faith were never truly saved by God in the first place, even if they demonstrated many of the fruits of faith. Everyone who has been saved by God will continue in faith until death.
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