The Greek verb atheteō comes from a- (not) + tithēmi (to place/set) and means to set aside, reject, treat as null and void, or frustrate. It describes the active rejection of authority — whether divine or human. It is used of rejecting Christ (John 12:48), setting aside the Law (Hebrews 10:28), rejecting God (1 Thessalonians 4:8), and nullifying grace (Galatians 2:21).
To athetein (set aside) the gospel is the gravest spiritual act. Galatians 2:21 is especially striking: I do not set aside the grace of God, for if righteousness could be gained through the law, Christ died for nothing! — Paul treats seeking righteousness by law as effectively nullifying Christ's death. To add human works to the gospel is to make the cross unnecessary. The writer of Hebrews warns with equal severity: Anyone who rejected the law of Moses died without mercy... how much more severely do you think someone deserves to be punished who has trampled the Son of God underfoot? (Hebrews 10:28–29).