Is the presence of Christians and followers of other religions in a Muslim country sufficient evidence that the message of Muhammad, peace and blessings be upon him, has reached them, and consequently, are they to be treated as disbelievers if they do not follow him?

Their presence among Muslims places them in the category of those to whom the message of Muhammad, peace and blessings be upon him, has reached. The rulings regarding that apply to them, based on the words of Allah the Almighty: "And this Qur'an has been revealed to me that I may warn you thereby and whom it reaches." (Quran 6:19) And based on the words of the Prophet, peace and blessings be upon him: "By the One in Whose hand is my soul, no one from this nation, neither a Jew nor a Christian, hears of me, then dies without believing in that with which I was sent, except that he will be among the people of the Fire."