What is a relapse, what are its causes and treatment, and what is the meaning of Ibn Al-Qayyim's statement: "Those who know Allah have unanimously agreed that sins committed in private are the root of relapses"?

A relapse in guidance means a change for the worse, like a penitent returning to their sins or an obedient person straying from righteousness. It is akin to physical ailments. Among its most important causes are: prolonged hope, hardness of heart, and neglecting to ponder divine revelation, which opens the doors to desires and doubts. Allah Almighty said: "Has the time not come for those who have believed for their hearts to become humbly submissive at the remembrance of Allah and what has descended of the truth? And let them not be like those who were given the Scripture before, and a long period passed over them, so their hearts hardened; and many of them were defiantly disobedient." Its treatment lies in identifying its cause.