Its by faith. Take the brother in 1 Cor 5, I think. Lets say that he was strong in faith even though, he committed sin that was wrong to even the Greeks, that of having his father's wife. The church there was instructed to shun him, but not to count him an unbeliever. In 2 Cor, the church is instructed to receive him back into fellowship, since he had repented of that sin. Now, the one who is weak in faith sees his brother eat food sacrificed to an idol and is emboldened in himself to also eat, but he doesn't have the understanding yet of only one God, being a babe in regards to the word. Maturity is for those who eat the strong meat of the word and by reason of use and are able to discern good and evil. So the weak brother became defiled in his own mind because he didn't eat from belief, he returned to the world and was overcome by his sin and fell away from the Lord as his faith, like the flame from a candle, went out. And this is the victory that overcometh the world, even our faith.
Or in 1 Jn 3 Hereby we know that we are of the truth and can assure our hearts before him. If our heart condemn us, God is greater than our heart and knoweth all things. Beloved, if our heart condemn us not then we have confidence before God. And whatsoever we ask we receive of him because we keep his commandments and do those things that are pleasing in his sight. And this is his commandment that we believe on his Son Jesus Christ and that we love one another as he commanded. Having faith in Jesus is the main commandment then we are to grow in love with those made in the Lord's image, but to believe in Jesus is the foundation of the new covenant.
The difference between stumbling and falling away is that when we stumble, we get back up and when we're overcome is when we don't get back up or draw near again to the Lord with a repentant heart while the flame of faith yet maintains its light.
For without faith, it is impossible to please him...