It is very simple.
God created man
Sent His message to appointed Prophets over the ages
After each message became lost, another Prophet would be chosen
Same message
Same God
Now down to the last Prophet, is M'hammed, ( peace be upon him and all )
All the prophets know eachother and form one brotherhood. They will be reunited in Paradise, the Gardens beneath which rivers flow, ( Garden of Eden )
The people choose to follow or not. They always have a choice.