Amongst the actions of kufr are those for which it is not necessary that a person make istihlaal of them (i.e. declare them to be lawful with his heart) before he be considered a disbeliever, outside the fold of Islaam.
Shaikh ul-Islaam Ibn Taymiyyah said, "And it is necessary for it to be known that the statement that 'the disbelief of the one who reviled (Allaah, the Messenger or the religion) is only disbelief because he made istihlaal of his act (i.e. declared this revilement to be lawful)' is a repugnant mistake and very great error...and this is from numerous angles..."
When a person reviles the Messenger (sallallaahu alaihi wasallam) or throws the Qur'an into some filth, or kicks the Qur'an without compulsion or out of error for example, then it is not a requirement that he makes or considers this act to be lawful (istihlaal) as a matter of belief before takfir is made upon him. Rather it is sufficient to known that he intended his act and did it wilfully, not being under compulsion or the likes.
As for Istihlaal (having the belief that something that Allaah has made unlawful is lawful), then it is of two types, that which is related to action, and that which is related to belief.
Shaikh Ibn Uthaimeen said, "Istihlaal is that a person believes that something that Allaah has made unlawful is lawful. As for Istihlaal of action, then we need to observe: If this Istihlaal (is related to something) that expels from the religion then a person becomes a disbeliever and apostate by it. So for example, if a person worked with usury (i.e. took or gave usury) without believing in its lawfulness, yet he persists in working with it, then such a one does not become a disbeliever because he did not declare it to be lawful. However, if he said, "Usury is lawful" and he intends by this the usury that Allaah has declared unlawful, then he becomes a disbeliever, since he is a denier (mukaddhib) of Allaah and His Messenger (sallallaahu alaihi wasallam). So in this circumstance, Istihlaal would be that of action and also that of belief, with his heart. However, the Istihlaal in action, we need to look at the action itself, is it something which in and of itself, expels from the religion or not? And it is known that consuming interest does not make a person a disbeliever, rather it is one of the major sins. However, if a person prostrated to an idol, then he becomes a disbeliever. Why? Because this act itself expels from the religion. This is the principle, however it is necessary for us to observe another condition, and this is that the person who made something lawful (by istihlaal) is not excused due to ignorance, for if he is excused due to ignorance, then he does not become a disbeliever."
So Istihlaal is of two types, that which is related to action and that which is related to belief. That which is related to belief is disbelief in all circumstances, regardless of whether this istihlaal was in relation to something that only reaches the level of minor kufr or something reaching the level of major kufr. For example, if one made fornication lawful as a matter of belief, or drinking and likes then he is a disbeliever. Likewise, if he made reviling the Messenger (sallallaahu alaihi wasallam) or the religion to be lawful, then regardless of whether he did the act or not, then he becomes a disbeliever by this istihlaal.
As for the istihlaal that is related to action, and this merely means that a person commits the act - in other words he makes it lawful for himself in terms of his action only, in the sense that he performs the act - then in such a situation one must look at the action he committed. Is it something that does not reach the level of major kufr? Such as fornication, stealing, gambling and the likes. In this case he is not a disbeliever by this Istihlaal of action. However, if it is an act (or statement) which expels from the religion, such as prostrating to an idol, then he becomes a disbeliever by this Istihlaal of action. And this is the principle concerning Istihlaal in the view of our Ulamaa. In light of this, the well known statement of Imaam at-Tahaawi, "We do not declare someone a disbeliever on account of a sin he committed, so long as he does not declare it lawful", is subject to the above clarification and is not taken absolutely.
With respect to Juhood, then Imaam Abu Ja'far at-Tahaawi said, "A servant does not exit from Imaan except by denial (juhood) of that which brought him into it." Shaikh Ibn Maani' commented, "He intends to refute the Khawarij and the Mu'tazilah by this, those who expel such a person from Imaan on account of a sin that he commited".