The main difference between backsliding and falling away is that backsliding refers to a sinning Christian who repents and returns to the Lord, while falling away refers to a Christian who completely rejects God and dies in their sins. Backsliding typically happens when someone is weak and doesn't resist the temptation to sin. They may know that they're sinning, but they rationalize it by telling themselves that they'll repent later. Falling away, on the other hand, generally happens when someone has known the Lord for a while and has had plenty of opportunities to repent, but they deliberately choose to reject Him.