The Greek verb anathematizo means "to curse with a solemn oath" or "to place under a ban." It appears in Peter's denial (Mark 14:71) and the Jewish plot against Paul (Acts 23:12).
The two significant uses of anathematizo reveal the gravity of invoking divine curse. Peter called down curses while denying Christ — a catastrophic failure that Jesus subsequently restored in the most tender way (John 21). The Jewish conspirators' oath to kill Paul came to nothing, overturned by God's providence. Even the most severe oaths are subject to divine sovereignty.