Are all believers afflicted by intrusive thoughts (waswasah)? And did great scholars like Ibn Taymiyyah and Imam Ahmad suffer from them? If so, how were they able to serve Islam with such great effort?

Waswasah (insinuating whispers) is from the plots of Satan, who strives to corrupt a believer's religion and worldly life. This applies to all servants, but people differ in the extent of their surrender to his whispers. Muslim narrated from Abdullah ibn Mas'ud that the Prophet (peace be upon him) said: "There is none among you but has his companion (Qarin) from the jinn assigned to him." They said: "Even you, O Messenger of Allah?" He said: "Even me, but Allah helped me against him, so he submitted (fa aslama), and he only enjoins me to do good." The word "fa aslama" is narrated with both a damma (u) and a fatha (a) on the 'lam' (ل). The fatha is preferred. It is interpreted either as surrender and submission, or as embracing Islam, which is the more apparent meaning. The Ummah is in agreement on the infallibility of the Prophet (peace be upon him) from Satan. The hadith (definition) contains a warning against the temptation and whispers of the Qarin. Satan strives to whisper most when a servant approaches prayer and worship. The important thing is to strive to free oneself from the plots and whispers of Satan.