Question: We saw many enter into tribulation, and we knew them with firm knowledge due to long companionship, sitting together, and fellowship in residence and travel; and we did not see from them except adherence to piety, good character, humility, modesty, benefiting brothers, general and specific mercy, good conduct with the near and the far… etc. So what is the cause, along with what we know of their good insight in religion, and the rest of what we mentioned?
Answer: The causes of tribulation away from religion are:
1- Disobedience to Allah—Exalted is He—for it is a cause of entering into tribulation. Allah—Exalted is He—said regarding the people of the Sabbath: "Thus We test them because they were defiantly disobedient" [Al-A‘raf], and He—Exalted is He—said: "Then the outcome of those who did evil was the worst, because they denied the signs of Allah and used to mock them" [Ar-Rum].
2- That there be in the heart pride, or zealotry, or partisanship, or arrogance whose effect did not appear on the person’s conduct and dealings, and the person had not been exposed to what stirs it and brings it out from his heart; then when Allah—Exalted is He—tests him with what stirs that and makes it appear, he falls into tribulation—such as the tribulation of Iblis, may Allah—Exalted is He—curse him.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.3