How did Adam marry off his children, and why were people divided thereafter into Jews and Muslims?

Allah permitted Adam to marry his male children to their sisters out of necessity. Thus, a male and a female were born to him in one womb, so he would marry the male from one womb to the female from another, and the female from one womb to the male from another.

All people trace their origin back to Adam, peace be upon him. The distinction among them lies only in fulfilling the purpose of their creation, which is to affirm Allah's oneness and worship Him, as Allah Almighty said: ﴿And I did not create the jinn and mankind except to worship Me.﴾ [Adh-Dhariyat:56]. Allah has made the bond of religion paramount over all other bonds, as He Almighty said: ﴿The believers are nothing but brothers.﴾ [Al-Hujurat:10], and He said: ﴿Do not take your fathers and your brothers as allies if they have preferred disbelief over belief.﴾ [At-Tawbah:23].