In regard to the person who does not possess Imaan, Allah Ta‘ala explains in the Quraan Majeed:

مَثَلُ الَّذِیۡنَ کَفَرُوۡا بِرَبِّہِمۡ اَعۡمَالُہُمۡ کَرَمَادِۣ اشۡتَدَّتۡ بِہِ الرِّیۡحُ فِیۡ یَوۡمٍ عَاصِفٍ ؕ لَا یَقۡدِرُوۡنَ مِمَّا کَسَبُوۡا عَلٰی شَیۡءٍ ؕ ذٰلِکَ ہُوَ الضَّلٰلُ الۡبَعِیۡدُ ﴿۱۸﴾

“The example of the deeds of those who deny their Rabb is like ashes blown away by the wind on a stormy day. They will not gain anything (any reward of the good works) of what they earned. That is the furthest point of straying away from the right path. (Surah Ebrahim v. 18)

In another verse of the Quraan Majeed, Allah Ta‘ala says:

قُلۡ ہَلۡ نُنَبِّئُکُمۡ بِالۡاَخۡسَرِیۡنَ اَعۡمَالًا ﴿۱۰۳﴾ؕ اَلَّذِیۡنَ ضَلَّ سَعۡیُہُمۡ فِی الۡحَیٰوۃِ الدُّنۡیَا وَ ہُمۡ یَحۡسَبُوۡنَ اَنَّہُمۡ یُحۡسِنُوۡنَ صُنۡعًا ﴿۱۰۴﴾ اُولٰٓئِکَ الَّذِیۡنَ کَفَرُوۡا بِاٰیٰتِ رَبِّہِمۡ وَ لِقَآئِہٖ فَحَبِطَتۡ اَعۡمَالُہُمۡ فَلَا نُقِیۡمُ لَہُمۡ یَوۡمَ الۡقِیٰمَۃِ وَزۡنًا ﴿۱۰۵﴾ ذٰلِکَ جَزَآؤُہُمۡ جَہَنَّمُ بِمَا کَفَرُوۡا وَ اتَّخَذُوۡۤا اٰیٰتِیۡ وَ رُسُلِیۡ ہُزُوًا ﴿۱۰۶﴾

“Say: Shall We inform you of those who are the greatest losers in terms of their deeds? (They are) those whose efforts have been wasted in this life while they thought that they were acquiring good by their deeds. They are those who used to deny the aayaat (proofs, verses of the Quraan Majeed and the signs of Allah Ta‘ala) of their Lord and the meeting with Him (in the Hereafter). So their deeds are in vain, and on the Day of Resurrection, we shall assign no weight for them (for the good deeds they did in the world). That shall be their recompense – Jahannum - because they disbelieved and took My signs and My Messengers as means of jest and mockery. (Surah Kahf v. 103 - 106)

From the above verses, we conclude that without Imaan, one will not be a believer.

Imaan is to believe in the fundamental tenets of faith such as the oneness of Allah Ta‘ala and believe in all His attributes, the finality of the prophethood of Hazrat Rasulullah (sallallahu ‘alaihi wasallam), the prophethood of all the Ambiyaa (‘alaihimus salaam), the existence of the angels, the revelation of all the divine scriptures, the belief of taqdeer (predestination), the occurrence of the Day of Qiyaamah and life after death, the existence of Jannah and Jahannum, etc.

If one merely performs salaah, discharges zakaat, fasts in the month of Ramadhaan, performs hajj, etc., but he does not bring Imaan in the fundamental tenets of faith, he will not be regarded as a Muslim. Therefore, it is imperative for one to study Islam correctly and learn the fundamental tenets and beliefs of Islam so that one may become a true believer and attain the rewards promised for good deeds, as well as acquire the closeness and proximity of Allah Ta‘ala.