Shaykh al-Islaam Ibn Taymiyyah (rahimahullaah) stated as occurs in Majmu' al-Fatawa (7/191,193):
إن الإنسان قد يعرف أن الحق مع غيره ومع هذا يجحد ذلك لحسده إياه ، أو لطلب علوه عليه ، أو لهوى النفس ، ويحمله ذلك الهوى على أن يعتدي عليه ويرد ما يقول بكل طريق ، وهو في قلبه يعلم أن الحق معه ، وعامة من كذب الرسل علموا أن الحق معهم وأنهم صادقون ، لكن إما لحسدهم وإما لارادتهم العلو والرياسة ، وإما لحبهم دينهم الذي كانوا عليه وما يحصل لهم به من الأغراض كأموال ورياسة وصداقة أقوام وغير ذلك ، فيرون في اتباع الرسل ترك الأهواء المحبوبة إليهم أو حصول أمور مكروهة إليهم ، فيكذبونهم ويعادونهم فيكـونون من أكفـر الناس كإبليس وفرعـون ، مع علمهم بأنهم على الباطل والرسـل على الحق
بل أبو طالب وغيره كانوا يحبون النبي (صلى الله عليه وسلم) ويحبون علو كلمته ، وليس عندهم حسد له ، وكانوا يعلمون صدقه ، ولكن كانوا يعلمون أن في متابعته فراق دين آبائهم وذم قريش لهم ، فما احتملت نفوسهم ترك تلك العادة واحتمال هذا الذم
A man may know that the truth is with another but alongside this he will reject (that truth) due to his envy of him, or due to seeking elevation (over him) or due to a desire of the soul. And that desire carries him to transgress against (that one) and reject what he says using every method whilst in his heart he knows (full well) that the truth is with him.
The generality of those who belied the Messengers knew that the truth is with them and that they are truthful but due to envy of them or desiring elevation and leadership or loving the religion they were upon and the objectives they realize through it such as wealth, leadership and friendship of (various) people and other than that, (due to these reasons) they see in the following of the Messengers (the necessity of) abandoning the desires that are beloved to them, or the arising of matters that are dislikeable to them. Hence, they belie them (with takdheeb) and show enmity to them and thus become the most disbelieving of people, such as Iblees and Fir'aun, alongside their knowledge that they are upon falsehood and the Messengers are upon the truth...
... Rather, Abu Talib and others used to love the Prophet (sallallaahu alayhi wasallam) and would love his word to be uppermost and they did not have any envy towards him and they knew of this truthfulness but they (also) knew that following him would entail separating from the religion of their forefathers, and the rebuke of Quraysh upon them. So their souls were not able to bear the abandonment of this habitual (path) and bear this rebuke (of them).
Mere acknowledgement of the truthfulness of the Messengers is not sufficient to enter a person into eemaan, since this is only the speech of the heart (ma'rifah, tasdeeq). It is not until this tasdeeq leads to the expression of the actions of the heart that faith settles in a person. However, alongside that tasdeeq, there may be desires that come in between that tasdeeq and the expression of the actions of the heart. Hence, jealousy, pursuit of leadership and wealth, or loss of them, or fearing the rebuke of one's family, tribe and the likes, or fearing the loss of pleasures that a person might otherwise enjoy and which faith might prevent him from, all of these make a person withhold from faith (eemaan) despite his knowledge that the truth is in what the Messengers said and that he is upon falsehood in the path he wishes to remain upon.
This indicates that kufr (disbelief) is not just the absence of tasdeeq (belief) and ma'rifah (knowledge) in the heart, but it is the absence of the actions of the heart and it is here where the battle lies between Ahl al-Sunnah and the Murji'ah as it relates to the topic of the reality of faith.