بسم الله الرحمن الرحيم
Definition of Fitrah

Allāh subhānahu wata`ālā said in the Qur'ān:
"Set your face to the deen in sincerity (hanîfan) which is Allāh's Fitrah (the nature made by Allāh) upon which He created mankind.  There is no changing the creation of Allāh. That is the right deen but most people know not."  (Ar-Ruum 30:30)
 
Thus, Allāh commands us to persevere in the religion which He (sunhānahu wata`ālā) has prescribed for us; the worship of Allāh alone, the religion of Ibrāhīm, to which Allāh has guided us and which He has perfected for us with the utmost perfection.  In this manner, we will also adhere to the sound fitrah with which He created His creation.  Allāh created His creation to recognize Him and know His Oneness (Tawheed), that there is no God except Him.
 
The sharee‘āt that the prophets were sent with are guiding lights to the essential faith in Allāh which is inherent in every human being.  Since this faith comes from Allāh, it naturally follows that only laws capable of guiding man back to it must also come from Allāh, hence Islām is called deen-ul-fitrah, the religion of human nature.

The prophet (sallAllāhu `alayhi wasalām) said: "Every new-born child is born in a state of fitrah.  Then his parents make him a Jew, a Christian or a Magian..." (Saheeh Muslim).  Although all children are born in a state of Fitrah, the influence of their environment is also a factor that affects their religion.  If there are no adverse influences, then the children will continuously manifest their Fitrah as their true nature.
 
Mankind is distinguished from the rest of the creation because we have been endowed with intellect (‘aql) and free-will (irādah).  The intellect enables us to discern right from wrong.  We can use these faculties to complement our Fitrah and to please Allāh or to be untrue to it and displease Allāh.  The choice is ours. 
 
Fitrah which is integral to man's spirit (rûh) was created by Allāh so that man will acknowledge Him as the Lord Who has power over all things.  Tawheed is intrinsic to man's Fitrah because Allāh in His infinite wisdom intended for man to know Him as the One God.  This is why man was able to acknowledge his Lord before his existence on earth, that is, in pre-existence state.
"And (remember) when your Lord brought forth from the Children of Ādām, from their loins, their seed and made them testify as to themselves (saying): "Am I not your Lord?'' They said: "Yes! We testify.''  Lest you should say on the Day of Resurrection: "Verily, we were unaware of this!''
Or lest you should say: "It was only our fathers aforetime who took others as partners in worship along with Allāh, and we were (merely their) descendants after them; will You then destroy us because of the deeds of men who practiced falsehood?'
Thus do We explain the Ayāt in detail, so that they may turn (unto the truth)."  (Al- A`rāf 7:172-174)
Allāh stated that He brought the descendants of Ādām out of their fathers' loins, and they testified against themselves that Allāh is their Lord and King and that there is no deity worthy of worship except Him.  Allāh created them on this Fitrah, or way.
 
The function of the prophets and Divine revelation is not only to remind man about that which he already knows (that is, Tawheed), but also to teach him that which he does not yet know (that is, sharee'ah).  Man already knows Tawheed because of the pre-existent Fitrah as well as his earthly unchanging Fitrah.  The prophets have come only to remind man of Tawheed; the choice is left to the individual, as suggested in the following verse:

"Surely, this is a reminder; so whoever wills, let him take a way to his Lord."  (Al-Insān 76:29)
 
Knowledge of the Divinely revealed laws, the methodology of worship and devotion, etc. are acquired by man from sharee`ah which is based on Divine revelation and the teachings of the prophets (`alayhimussalām).  Since every individual is endowed with the innate knowledge of Tawheed, he is held accountable for his belief in Allāh because of his Fitrah.  Not every soul, however, will be held accountable for not practicing sharee`ah because knowledge of sharee`ah is acquired only by those who received the message of the Divine revelations and the teachings of he prophets (`alayhimussalām).