To declare one’s faith is also an article of faith
The person who believes and does not verbally declare it without any legitimate reason that person is not a believer with Allah Most High. In the opinion of those who hold that verbal declaration is also an article of faith, that person is destined for hell. Imam Fakhr Ul Islam and Shams Ul Aimma and most jurists have held this opinion.
However, in the opinion of those scholars with whom verbal declaration is not an article of faith, such a person is a believer, but with Allah Most High the laws of the Shariah don't apply to that person. This situation is the opposite of a hypocrite. A hypocrite (Munafiq) is not a believer with Allah Most High but people consider him as believer.
It is written in Sharh Maqasid that this difference of opinion is when a person can talk and his lack of verbal declaration is not due to denial (of Allah Most High and His Prophet). However if a person believes in Islam and (for some reason) did not get the time to verbally declare it, then everybody agrees that that person is by consensus a believer.
Mulla ‘Ali Qari (May Allah shower His Mercy on him) has written in Sharh Shifa that saying that such a person whose belief (in Allah Most High and His Prophet) was established but could not verbally declare it is not a believer is a weak opinion. Of course, if such a person had enough time to verbally declare his (or her) belief and was also asked to do so but refused, then by consensus such a person is not a believer. In fact, that person is a disbeliever due to his stubbornness and disobedience.
Thus, we learn from this discussion that verbal declaration is also an article of faith. However, it should be remembered that faith is essentially an affirmation of the heart. It is clear that the tongue is a means of expressing one’s thoughts. For this reason, a verbal declaration will be the proof of whether one has faith or not. In light of this, verbal declaration is an article of faith which is sometimes waived.
Thus when one is free to do so, verbal declaration is an article of faith and when is not under any kind of duress, an absence of verbal declaration will amount to absence of faith.
A verbal declaration being an article of faith in this manner does not negate that faith is essentially an affirmation. Those scholars with whom verbal declaration is an article of faith, it is in this sense.
[Excerpted from Noor Ul Masabeeh, Vol. 1, Kitaab Ul Imaan]