What is the meaning of: "The ambushes of hearts appear during tribulations"?

The meaning of the phrase is that for one who has been nurtured on virtues such as truthfulness and obedience to Allah and His Messenger, their effects become apparent during times of tribulation. Such a person prioritizes obedience to Allah and His Messenger over all else. Similarly, for one who has been nurtured on vices such as lying and fanaticism, these traits will also become apparent during times of tribulation. Ibn Taymiyyah clarifies that it is incumbent upon the believer to love what Allah and His Messenger love, and it is not permissible for anyone to make a person the origin of religion except for the Messenger of Allah, peace be upon him, nor to follow anyone but the Book of Allah. Thus, whoever gives allegiance and enmity based on agreement with a person, whoever that person may be, is "among those who divided their religion and became sects." A person should accustom themselves to inner understanding and acting upon it, for this is a deterrent, and "the hidden secrets of hearts become manifest during times of tribulation."