Assalamu Alaykum wa ramatullaahi wa barakaatuh.
Is it true that everybody in this world was born as a Muslim but it depends on the religion he or she met is parent doing?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Allah created all people and endowed them with the predisposition to believe in Allah alone and to submit to Him. There is no new-born child that is not born with the Fitrah (natural predisposition in believing in Allah). Some people stay with their Fitrah, and some are misled by devils among mankind and jinn.

The Prophet said in a Qudsi Hadeeth: “Allah says: I created all My Servants Hunafaa’ (i.e., following the original religion of monotheism), then the devils misled them from their religion and forbade them from what I made lawful for them and commanded them to associate with Me that which I have not sent down any authority for.” [Muslim]

The Prophet also said: “Every child is born with the Fitrah, but his parents make him a Jew, Christian or a Zoroastrian.” [Al-Bukhari and Muslim]

An-Nawawi said: “Allah’s saying "I created all My servants Hunafaa’": i.e. Muslims; but it was also said: they are pure from sins; and it was said: they are straight (on the Right Path), turning to Allah and accepting guidance … Then the devils misled them, so they took them away from what they followed and led them to falsehood.”

Ibn Katheer said about the interpretation of the words of Allah (which mean): "{So direct your face 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.}: Allah says: so set your face and persevere in the religion which Allah has prescribed for you, the worship of Allah Alone, the religion of Ibraaheem (Abraham), to which Allah has guided you and which He has perfected for you with the utmost perfection. In this manner, you will also adhere to the sound Fitrah with which He created His creation. Allah created His creation to know Him and His Oneness, and to know that there is no God except Him."

He also said, "Allah created all of His slaves equal in regard to the natural Fitrah, and no one is born without this sound Fitrah, and there is no difference among people regarding this.” [End of quote]

Allah Knows best.