Ibn Kathir states,
"Ikrimah, Mujahid, Hasan, Dhihak and Qatadah said in explanation of the aforementioned ayah the fitrah of Allah refers to the din of Islam and Allah's creation refers to Allah's din. Imam Ibn Kathir goes onto say the fitrah of Allah refers to knowledge of Allah and Tawhid ie all of creation are created upon the knowledge of Allah and his Tawhid. (Tafsir Ibn Kathir (6/313)
This is further supported by the hadith, every child is born upon natural disposition ie fitrah and then his parents convert him to Christianity, Judaism or a fire worshipper. (Bukhari 4775, Muslim 2658).
As a side point those who deny the idea of uzar bil Jahal use this as one of the evidences in support.