It is a proven fact from the Holy Quran and Ahadith that the parents of the Holy Prophet (sallal laahu alaihi wasallam) - Sayyiduna Abdullah and Sayyidah Aamina (radi Allahu anhuma) - were always on Iman and left this world with Iman.
Just as the cities of Makkah and Madina are the most exalted cities in the world due to being the birth place and place of demise of the Prophet (sallal laahu alaihi wasallam), likewise, the genealogy of the Holy Prophet (sallal laahu alaihi wasallam) is the most exalted and pure chain in the Universe.
All his forefathers, from his father, Hazrat Abdullah (radi Allahu anhu), to Hazrat Adam (alaihis salaam), were Mu'min, Muwaahids (believers in the Oneness of Allah), Aabids and Zaahids. None from amongst them were idol-worshippers or Faasiqs (sinners).
Actually, the Noor of Rasoolullah (sallal laahu alaihi wasallam) travelled from Hazrat Adam (alaihis salaam) to Hazrat Abdullah (radi Allahu anhu), in the family chain of those who always sincerely worshipped One Allah.
For those who say, Ma'az-Allah, that the parents of the Prophet (sallal laahu alaihi wasallam) were non-believers, they should think and ponder that if the parents of no other Prophet were non-believers, how then would the parents of the greatest of all Prophets be non-believers?