In the Prophet’s time, there was a well demarcated line that determined whether or not you were a Kafir (Rejector). If you believed that Muhammad (pbuh) was the messenger of God, but you still persisted in worshiping Idols (as was the prevalent form of religion then), then you were a Kafir. Further, most of the time, the Kafirs mentioned in the Quran referred to those that actively resisted the Prophet’s mission. It was for them that everlasting Hell was promised in the next world.
What constitutes Kufr in these days then? The well demarcated line during the prophet’s time is much more blurred now. We even have Muslims fighting each other these days because each party believes the other is spreading untruth and hence is in this respect denying the Prophet.
Kufr actually means to reject the Truth. However, every person guilty of ‘kufr’ is not necessarily a ‘Kafir’, for the simple reason that ‘rejection of Truth’ can effectively take place for a number of genuine reasons, as, for instance, ignorance or misunderstanding etc. On the contrary, ‘kafir’ is a person who rejects the truth after having fully recognized it and after the Truth has become clearly distinct from falsehood for him/her.
When a messenger of God is present among his addressees, God distinguishes the Truth – including the truthfulness of the messenger himself – from falsehood and, as a result, those who persist in rejecting the Truth are declared Kafirs– i.e. rejecters. Furthermore, at the time of the advent of a messenger of God, there is a live link between the humankind – through the selected prophet of God – and God. As a result, at such times there can be a few aspects of God’s knowledge which are brought into the open, which under other circumstances remain hidden from ordinary human beings like us. Whether or not a person has rejected something after having fully recognized it as God’s truth is only known to the All-Knowing God. No one other than God can decide about this phenomenon, which is clearly internal to every man.
In view of the foregoing, at present times, we do not consider it correct for any human being to declare another a ‘Kafir’. Only God, with His absolute knowledge about the motives, intentions and all other hidden thoughts and phenomena, can declare an individual or a person to be ‘kafir’. However, there is one exception to this principle. This exception relates to a person’s declared adherence to what he, himself, understands and declares to be ‘kufr’.
One point which is extremely important to understand is that there is a difference between labelling someone as ‘kafir’ or ‘non-muslim’.Whilst collectivity of muslims, not individuals or groups, can label a group as non muslim based on their extreme beliefs but no group can be called ‘Kafir’.
Hope it helps!