Why did God not create all human beings as Muslims, and does pondering this question expel a Muslim from the faith?

Allah created people with an innate disposition (fitra) for monotheism and took a covenant from them before their creation. He clarified for them the paths of good and evil and left them the freedom to choose. Out of His perfect mercy, He sent messengers to them as bringers of glad tidings and warners, so that a proof might be established against them. Some people responded to the messengers, while others followed the devils. Allah possesses profound wisdom in the disparities and differences among people. Had He willed, He could have made them one nation, but He guides whom He wills and misleads whom He wills, so that trials may occur and servitude (to Him) may become manifest.