Fitra, or fitrah (Arabic فطرة), is an Arabic word meaning ‘disposition’, ‘nature’, ‘constitution’, or ‘instinct’. The word fitrah comes from the Arabic radicals fa ta ra. The root action means, he clove, split, slit, rent or cracked it.
Any individual who looks into themselves and consults their conscious will find that there is nothing in Islam which contradicts that which is naturally understood to be right or wrong. Allah (Glory be to Him) says in Qur’aan: Allah's Fitrah (i.e. Allâh's Islâmic Monotheism) with which He has created mankind. No change let there be in Khalq-illâh (i.e. the religion of Allâh – Islâmic Monotheism), that is the straight religion, but most of men know not. Qur’an. Surah Rum 30:30, Tafsir Ibn kathir. 
When a child is born, it has with it a natural belief in Allah (Glory be to Him). This natural belief is called in Arabic the “Fitrah”. Fitrah literally means nature, natural disposition and innate character. In Islam, it refers to the natural inclination towards good and aversion towards evil.
The term fitrah literally means, creation; the causing a thing to exist for the first time; and the natural constitution with which a child is created in his mother’s womb. It is for this reason that Muslims prefer to refer to those who embrace Islam as reverts rather than converts, as it is believed they are returning to a perceived pure state.
“[Iblees said] ‘…and indeed I will order them to change the nature created by Allah.’” [al-Nisa’ 4:119] Tafseer Ibn Katheer
Islam is also called deen al-fitrah, the religion of human nature, because its laws and its teachings are in full harmony with the normal and the natural inclination of the human fitrah to believe in and submit to the Creator. 
One aspect of the Fitrah is the belief in One God, therefore it is common to see people in every community who believe there is just One God. No one ever told or proved that there is only one Creator, but this is what the “Fitrah” of every human is! This is just natural therefore, Islam is the natural way of life.
Islam is also called dîn al-fitrah, the religion of human nature, because its laws and its teachings are in full harmony with the normal and the natural inclination of the human fitrah to believe in and submit to the Creator. Like the word al-Islam, the word dîn also means, according to Lane, obedience and submission, among other meanings. Allah states:
‘And who is better in obedience (in dîn) than he who resigns himself to Allah?’ (Qur'an 4:125)
‘There shall be no compulsion in obedience (dîn).’ (Qur’ân 2:256)
Ad-deen implies religion in the widest sense of the word, embracing both the practical aspects of the acts of worship and ordinary transactions of life, and the teachings of religion; and it is a name for that whereby one serves Allah.
‘Truly, the religion (dîn) in the sight of Allah is al-Islam.’ (Qur’ân 3:19) 
Islamic monotheism is described as the religion of fitrah – that of the inherent nature of mankind. Allah says: “So direct your face [i.e., self] toward the religion, inclining to truth. [Adhere to] the fitrah of Allah upon which He has created [all] people. No change should there be in the creation of Allah. That is the correct religion, but most of the people do not know.” Quran.Surah Rum 30:30.
Hadith: ‘And if his parents are Jews, they make him a Jew, with respect to his worldly situation; [i.e. with respect to inheritance, etc.] and if Christians, they make him a Christian, with respect to that situation; and if Magians, they make him a Magian, with respect to that situation; his situation is the same as that of his parents until his tongue speaks for him; but if he dies before his attaining to the age when sexual maturity begins to show itself, he dies in a state of conformity to his preceding natural constitution, with which he was created in his mother’s womb.’ Sahih Al Bukhari Vol 6:298, Sahih Al Muslim 6426. 
As a new born child’s body submits to the physical laws of nature, its soul naturally submits to the fact that Allah (Glory be to Him) is its Lord and Creator. It is the surrounding, the parents, the environment and the society which moulds the child in its own way and the child helplessly accepts it and imbibes the beliefs of its surroundings due to his physical weakness.
“Every child is born on the Fitrah but it is his parents who make him a Jew or a Christian.” Sahih Al Muslim 6426,Muwatta Imam Malik, 16:53; Sahih Al Bukhari, 6: 298 
…Man is distinguished from the rest of the creation because he has been endowed with intellect (‘aql) and free-will (irâdah). The intellect enables him to discern right from wrong. He can use these faculties to complement his fitrah and to please Allah or to be untrue to it and displease Allah. The choice is his. The prophets and Divine revelation are external sources of guidance to guide the intellect and will of man. The Qur'aan declares that the Prophet, may Allah bless him and grant him peace, enjoins the right and lawful things (ma‘rûf) and forbids the wrong and unlawful things (munkar). Man is responsible for his actions and accountable to Allah for every atom of right and wrong that he does. It is in this sense of accountability that guides man to act in accordance with the Divine will. It empowers him to struggle against the wrong-doing of his lower self (nafs) as well as the negative influences of the social circumstances. The central hadith makes plain that it is the social circumstances after the birth of the child that causes the individual to diverge from fitrah. Hence if someone follows an aberrant path it is not because of any innate wrong within his nature, but because of the emergence of the lower self or nafs after birth, and negative effects in the social circumstances. 
Dr Justin Barrett, a senior researcher at the University of Oxford's Centre for Anthropology and Mind, claims that young people have a predisposition to believe in a supreme being because they assume that everything in the world was created with a purpose. He says that young children have faith even when they have not been taught about it by family or at school, and argues that even those raised alone on a desert island would come to believe in God.
"The preponderance of scientific evidence for the past 10 years or so has shown that a lot more seems to be built into the natural development of children's minds than we once thought, including a predisposition to see the natural world as designed and purposeful and that some kind of intelligent being is behind that purpose," he told BBC Radio 4's Today programme.
"If we threw a handful on an island and they raised themselves I think they would believe in God." In a lecture to be given at the University of Cambridge's Faraday Institute on Tuesday, Dr Barrett will cite psychological experiments carried out on children that he says show they instinctively believe that almost everything has been designed with a specific purpose.
Dr Barrett said there is evidence that even by the age of four, children understand that although some objects are made by humans, the natural world is different. He added that this means children are more likely to believe in creationism rather than evolution, despite what they may be told by parents or teachers.
Dr Barrett claimed anthropologists have found that in some cultures children believe in God even when religious teachings are withheld from them. "Children's normally and naturally developing minds make them prone to believe in divine creation and intelligent design. In contrast, evolution is unnatural for human minds; relatively difficult to believe."
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