What separates faith from disbelief?

Correctly situating the boundary that separates faith from disbelief is one of the most important of all theological issues. Faith (Iman) signifies 'confirmation' (tasd'iq), while disbelief (kufr) literally signifies 'covering over'; hence, one also calls a farmer a kafir, that is, one who 'covers over' the seed of wheat by the earth. But in theological parlance, faith signifies belief in the Oneness of God, in the Day of Judgement, in the message of the last Prophet; and, of course, belief in the message of the Prophet of Islam implies acknowledgement of the prophecies brought by all previous Prophets and revealed books, while following all the doctrines and rulings brought by the Prophet of Islam.
The true locus of faith is the heart of man as the Qur'an says:

... Those - He has decreed within their hearts faith and supported them with spirit from Him. (Sura al¬Mujadila, 58: 2 2).

In theological parlance, faith signifies belief in the Oneness of God, in the Day of Judgement, in the message of the last Prophet; and, of course, belief in the message of the Prophet of Islam implies acknowledgement of the prophecies brought by all previous Prophets and revealed books, while following all the doctrines and rulings brought by the Prophet of Islam.

Also, in relation to the bedouin of the desert who submitted to the power of Islam, but whose hearts were devoid of the light of true faith, the Qur'an says:

The bedouins say, "We have believed." Say, "You have not [yet] believed; but say [instead], 'We have submitted,' for faith has not yet entered your hearts. (Sura al-Hujurat, 49:14)

But of course, one can only form some kind of judgement about the faith of a person if the person in question positively expresses his faith, or the lack of it, either by way of speech or other means. The Qur'an says in regard to those who disavow their faith:

And they rejected them (our signs), while their [inner] selves were convinced thereof, out of injustice and haughtiness. So see how was the end of the corrupters. (Sura al-Naml,27:14)

Whenever a person denies the Oneness of God, or the Day of Judgement, or the Message of the Prophet, evidently, such a one will be judged an unbeliever, since denial of one of the axioms of the religion, implying therewith denial of the integrity of the Message, is tantamount to disbelief .