You have asked an important question.
If you examine the messages of the Prophets mentioned in the bibles and the Quran. You will find one thing that is continuous which is that they call to the worship of one God without ascribing partners to God. Then the follower affirms the prophet-hood of the Prophet of their time.
The differences came after, not with, the initial message. You see the first rejection was the Jews rejecting Jesus. When Muhammad (peace and blessings of God be upon) came many Jews and Christan's excepted Islam at his hands and others rejected.
Islam affirms all the previous messages and their laws were valid for a time and a place. Whereas Islam is final message of monotheism for mankind coupled with the final book and the final messenger.
So the universal belief is to believe in God and a messenger of that time. So who is the valid messenger of this time. Karen Armstrong (non-Muslim) has written a book called Muhammad : a Prophet of our time. See linkhttp://www.amazon.co.uk/Muhammad-Prophet-Time-Eminent-Lives/dp/0007232489/ref=pd_bbs_7?ie=UTF8&s=books&qid=1206306836&sr=8-7
Also Muslims believe in the Fitra or the primodal state of man which is to believe in God. This is inbuilt into the human condition and it can be hidden by denial but remains intact inside him.
Just to recap the universal belief is monethism and Prophethood which can be summed up the muslim creed as: "There is no deity except God and Muhammad is the Messenger."