The Arabic word Muslim comes from the root s-l-m which means to Submit or Surrender . So, Mu-slim means One who submits his will to Allah1. Every child when he/she is born , he/she is born in Islam i.e. Submission to God. Later due to the influence of parents, elders, teachers etc. the child may follow a path which is wrong.
Allah Almighty says in the Noble Qur'an :
“So direct your face toward the religion, inclining to truth. [Adhere to] the Fitrah2 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.”
~Surah Rum (30:30)
The Messenger of God , Muhammad (peace be upon him) said :
“No baby is born but upon Natural Religion (i.e. Islam , Submitting his will to Allah). It is his parents who make him a Jew or a Christian or a Polytheist3. A person said: Allah's Messenger, what is your opinion if they were to die before that (before reaching the age of adolescence when they can distinguish between right and wrong)? He said: It is Allah alone Who knows what they would be doing.”
~[Saheeh Muslim :: Book 33 : Hadith 6426}
A question that comes to mind of almost all non-muslim theists (Believers in God) is that,"What is my mistake if I took birth in a non-muslim family and Islam is the truth ? Why was'nt I born a Muslim ."I would like to reply to the second part first,
Everybody was born a Muslim (Submitter to God) according to Islam because our creator is not unjust in the least degree.
Now, coming to the first part “What is my mistake if I took birth in a non-muslim4 family?”
Only Islam has an answer to this type of question and a very clear cut reply from the Noble Qur'an is:
“...Had Allah willed, He would have made you one nation [united in religion], but [He intended] to test you in what He has given you; so race to [all that is] good. To Allah is your return all together, and He will [then] inform you concerning that over which you used to differ.”
~Surah Maidah (5:48)
The greatest example of absolute submission to God is the way in which a Muslim worships his creator, the way Prophet Jesus worshiped, the way Prophet Moses worshiped, the way Prophet Abraham worshiped and the way all the other Prophets worshiped God.
Allah also promises us to show the signs of truth of Islam:
“We will show them Our Signs in the universe, and in their ownselves, until it becomes manifest to them that this (the Quran) is the truth. Is it not sufficient in regard to your Lord that He is a Witness over all things?”
~[Surah Fussilat (41:53)]
The most important message of Islam is pure monotheism and Allah almost everywhere in the Noble Qur'an repeats this message in different forms. God will never forgive polytheism So it’s my request to the reader to Worship the creator, not the Creation.
Come back to the Natural Religion you were born in. This is a reminder for you and Allah promises to shows his signs to everyone so that they may believe but it’s up to you to believe and be among the people of Paradise or disbelieve and be among the people of Hellfire.
1- “Allah” is the Arabic word for God i.e. the One creator of the Universe and Everything
2- Islam is also called “Deen ul-Fitrah” i.e. Natural (innate) religion in which God created everyone.
3- Polytheism is the worship of multiple gods instead of worshiping the One true Almighty God.
4- Just being born in a Muslim family is not a ticket to paradise. For La Ilaha Illallah “There is nothing worthy to be worshiped except God” has a meaning and one must fulfill the commands of his Creator.