Who is a Kāfir? How can a Christian be a Kāfir and still be from among the People of the book?
A person becomes a Kāfir when he denies the truth in spite of being convinced that it is the truth. Since it is humanly impossible for a person to determine whether some person is denying the truth or not, it is only on the basis of information provided by the Almighty that a person can be called a Kāfir. In the times of His Rusul, He chose to impart this information to his Rusul through Wahī; however, after the departure of the last Rasūl Muhammad (sws), people who have knowingly denied the truth cannot be pinpointed since the institution of Wahī has been terminated. No Muslim preacher is in a position to reveal the truth in a manner a Rasūl is able to, nor can he ascertain who among his addressees is guilty of knowingly denying the truth. After the Prophet (sws), only on the Day of Judgement will it now be known whether a particular person is a Kāfir or not.
It is evident from this explanation that the Christians and Jews and followers of other religions in times after the Prophet (sws) are not Kāfirs; the right name for them is non-Muslims. As far as Christians are concerned, it is to be noted that they are basically followers of monotheism. They never admit to polytheism, though they are involved in it. A person becomes a polytheist when he openly admits that he is a polytheist, even though he may be practising polytheism in some form; the reason is that a person might be doing something wrong without realizing what he is doing; Christians, whether of today or from the period of Jesus (sws), have never admitted to polytheism. Trinity to them is in accordance with monotheism. Of course, we do not agree with them, but unless they realize it, we can only say that in spite of claiming to be monotheists they are involved in polytheism. Their case is the case of a Muslim who goes to the grave of a saint to ask him to grant a wish; we shall not call such a Muslim a polytheist; we shall tell him that what he is doing is something against monotheism to which he himself strongly claims adherence. Similarly, we shall not call Christians polytheists, but we will keep telling them that what they are doing is not in accordance with monotheism.
It is precisely for this reason that the Qur’ān never referred to the People of the Book as polytheists though they subscribed to certain blatant forms of polytheism. The Qur’ān only called the Ismaelites as polytheists because they admittedly subscribed and testified to the creed of polytheism. They strongly advocated that polytheism was the very religion the Almighty had revealed and claimed that they were the strong adherents to this religion.