A person who does not grasp the Grace of the Lord and receive it unconditionally can fail of the Grace of the Lord - i.e. they never enter into Grace.
You cannot fail of the Grace of the Lord by committing a sin. Otherwise, everyone has failed of the Grace of the Lord.
Satan attacks through the lust of the flesh, the lust of the eye and the pride of life. bitterness is the absence of love and peace. When a person stays InChrist His HolySpirit keeps them sweet.
The whole book of Hebrews is dedicated to the Jews who were still struggling between Moses and Jesus. Paul makes every attempt to elevate the sacrifice of Christ once and for all so the Jews relegate animal sacrifice to the past.
He emphasizes the priesthood of Christ Jesus and the importance of faith in His finished work so that performance becomes a byproduct of faith whereby we are accepted by the Lord rather than a means for being accepted by the Lord.
Sin is still an issue in the world - both for sinners and for believers. So, the warning to abstain from sin is so that you do not have to endure the chastisement that it brings.