"... and by the soul and He who perfected it! Then He inspired to it (the ability to understand) what is good for it and what is evil for it. Successful is he who purifies it, and failure is he who corrupts it." (91:1-10)
Almighty Allāh has made our souls such that we are able to distinguish between what is good and what is evil. But for human soul to function on its fitra, there is a condition--it must be kept pure, it must be immunized against spiritual corruption.
The soul is like a bulb which can give light provided it itself is not surrounded with a thick cover or dust; every human being has that light in his soul; however, those who keep it pure can enlighten their path with it while those who allow the `spiritual dirt' to gather upon it cannot see the path towards Allāh. (Incidentally, kufr (infidelity) literally means a cover, and so it implies that kufr prevents the inner light from showing the right path.)
The Prophet of Islam emphasized the same point when he said, "Every child is born with the believing nature (al-fitra),it is his parents who make him into a Jew or a Christian."2
Besides this fitra, Allāh has also provided us with various means to know Him and believe in Him; He sent prophets and messengers, He revealed the scriptures, and above all He created thousands of signs in the nature which remind us of Him.
"Soon We shall show them Our signs on the horizon (āfāq) and in themselves (anfus), until it becomes clear to them that this is the Truth." (41:53)
2. Al-Kulayni, al-Usul mina 'l-Kāfi, vol. 2, p. 13; al-Bukhāri,Sahih, vol. 2 (Beirut: Dār al-Fikr, 1401) p. 104; for its Arabic edition with English translation, see vol. 2 (Beirut: Dār al-`Arabiyya) p. 262.