Question:
What sin is so great that it will make us lose our salvation? What about someone who chooses to turn from God?

Answer:
The Lord Jesus says that if we are saved “no man” (person) can take us out of the Lord’s and the Father’s hands—this includes myself  is not able to remove me from their hands (John 10:27-30)!

The Scriptures tell us that if a person merely “tastes” but does not swallow “the heavenly gift” (Hebrews 6:4-6), and has “knowledge” of the Lord but has not believed on Him in his heart, “The latter end is worse with them than the beginning” (2 Peter 2:1921). So it is a serious thing to merely profess we have received Christ as our Saviour, but have not!
Nothing we or anyone else may do can change the value God places on the work Christ did on the cross to save our souls. If a true Christian turns away from God, God must deal with him as a son. If my son sins, he is still my son. “Whom the Lord loveth He chasteneth” (Hebrews 12:6; 1 Corinthians 5:1, 5).
“The Lord knoweth them that are His” (2 Timothy 2:19). We don’t always know who is the Lord’s for we can’t see the heart. We only see the “fruit” or actions (Matthew 7:20).
If we mess up, Philippians 1:6 is a help: “Having confidence of this very thing, that He who has begun in you a good work will complete it unto Jesus Christ’s day.” He won’t let us go. He has chosen us (Ephesians 1:4), and though He may have to chastise us, He will finish the “good work” He has begun in us.If we don’t have eternal security, we can’t grow spiritually, and we can’t enter into the privileges God gives to each Christian. If we are not sure we are saved, how can we thank the Lord for our salvation? How can we worship Him? How can we tell others of His good news? Also, if we are not saved, Satan couldn’t cause us to doubt our salvation.