How do diseases of the heart afflict believers, given their destructive nature, and why do they not initially affect those whom Allah guides, and did the Salaf warn against them and avoid them?

The core of heart diseases revolves around corrupt knowledge and corrupt intention, both of which stem from the trials of doubts and desires. These trials distinguish the truthful from the liar, as Allah the Almighty says: "Do people think that they will be left alone because they say, 'We believe,' and will not be tested? But We have certainly tested those before them, and Allah will surely know those who are truthful, and He will surely know the liars." The righteous predecessors recognized the gravity of heart diseases, feared hypocrisy for themselves, and prioritized sincerity. Indeed, the Messenger of Allah, peace and blessings be upon him, said: "Verily, in the body there is a piece of flesh which, if it is sound, the whole body is sound, and if it is corrupt, the whole body is corrupt. Indeed, it is the heart."