What is the limit of a believer's degrees (or ranks)?

The Qur’an and Sunnah (definition) indicate that believers vary in rank. They are, in order: Prophets, then the truthful (Siddiqoon), then the martyrs (Shuhadaa’), then the righteous (Salihoon). A truthful person (Siddiq) is one who is truthful in their words and actions, and who affirms the truth with their words and deeds, submitting to Allah and His Messenger. The ranks of the truthful vary because faith increases with obedience and decreases with disobedience. A believer either progresses or regresses; there is no standstill in the ranks of faith.