Al-Rum (The Romans) Sura 30: Verse 30
"Set thy face to religion as a hanif, in the primordial nature from God upon which He originated mankind — there is no altering the creation of God; that is the upright religion, but most of mankind know not."
To set, "turn," or "submit" one's face to or toward religion means to orient one's whole being toward worship and obedience to God. The present verse is thus taken by most commentators to be a command to follow the religion for which God created human beings. It is addressed directly to the Prophet or to all who hear the message. Some commentators take the first phrase to mean, "Follow the religion as a hanif and follow the primordial nature in which God created you". In another interpretation, primordial nature (fitrah) modifies religion, which is referred to as "primordial" because human beings were originally created for religion, as in 51: 56: I did not create jinn and mankind, save to worship Me.
As a hanif is understood by most to mean in a straight and upstanding manner, neither inclining nor adhering to past religions that have been altered or abrogated. Hanif is usually employed in the Quran with reference to Abraham, but in general, it indicates one who inclines away from misguidance and toward belief in the Oneness of God. To be truly devout (hanif) and incline toward the worship of God and away from idolatry is to live according to one's primordial nature (fitrah), in which all human beings have been created. One cannot change this underlying nature as a servant or worshipper of God, because there is no altering the creation of God. This phrase is also understood to mean that there is no change in God's religion; that is, there is no change in the substance or universal truths of religion, only in the forms in which these truths are revealed in different religions.
The reference to the fitrah is read by some to mean that human beings are born for Islam, so that anyone who follows any other religion is "astray" or "misguided". But al-Qurtubi maintains that it is impossible for the fitrah mentioned here to be Islam in its particular sense, because "Islam (submission) and iman (faith) are declaring with the tongue, embracing with the heart, and performing with the limbs," implying that if fitrah pertains to the original human nature, which is related to the spirit, it cannot pertain to the specific practices of a particular religious tradition because these can only be performed while a spirit resides in a body in this world. From this perspective, the upright religion could refer to religion as such and thus to any religious practice that accords with the fitrah. Nonetheless, most interpret upright religion as a reference to Islam in particular.
"The Study Quran: A New Translation and Commentary" - Seyyed Hossein Nasr