The recent question ” what makes the narrow path hard?” brings up an interesting issue for me. If someone is saved by understanding only God’s Grace can make it possible for them to be saved, then falls into the grace/works mindset that is so common in today’s Church, have they left the narrow way, and will they end up being lost?
If that ever happened, it would violate a central provision of OSAS. According to 2 Cor. 1:21-22 when we’re saved, it is God who makes us stand firm in Christ. He anointed us, set His seal of ownership on us, and put His spirit in our hearts as a deposit guaranteeing what is to come.
If the person in your example was really saved, his salvation would have been guaranteed at the moment of belief (Ephesians 1:13-14). If he fell into a grace / works mindset the Lord would go after him and bring Him back, like a shepherd retrieves a wandering sheep. If not, then departing from the narrow way wouldn’t matter because he wouldn’t have been saved in the first place.