Allah's Messenger (peace and blessings of Allah swt be upon him) said, "Every child is born with a true faith of Islam (i.e. to worship none but Allah Alone) but his parents convert him to Judaism, Christianity or Magainism, as an animal delivers a perfect baby animal. Do you find it mutilated?"
Then Abu Huraira recited the holy verses: "The pure Allah's Islamic nature (true faith of Islam) (i.e. worshipping none but Allah) with which He has created human beings. No change let there be in the religion of Allah (i.e. joining none in worship with Allah). That is the straight religion (Islam) but most of men know, not." (30.30)
Sahih Al-Bukhari Vol. 2 : No. 441