The Qur'aan shows, in no uncertain terms, that the message and the top priority of all the Prophets was one and the same:
"And verily, We have sent among every Ummah (nation, community) a Messenger (proclaiming): "Worship Allah (alone)", and avoid (or keep away from) Taghut5 (all false deities)." [al-Qur'aan, an-Nahl(16):36]
Muhammad ibn `Abdillaah (s), the last Prophet to be sent to the mankind and the Jinn, was ordered by Allah (swt) in the Qur'aan to say:
"Say you (O Muhammad): 'This is my way; I invite unto Allah (i.e. to the Oneness of Allah - Islamic Monotheism) with sure knowledge, I and whosoever follows me (also must invite others to Allah i.e. to the Oneness of Allah - Islamic Monotheism) with sure knowledge. And Glorified and Exalted be Allah (above all that they associate as partners with Him). And I am not of the Mushrikoon (polytheists; those who worship others along with Allah or set up rivals or partners to Allah).'" [al-Qur'aan, Yusuf(12):108]
This verse makes it clear that the Last Prophet and whosoever follows him call to Allah first and foremost. As Ibn Katheer explains, calling to Allah means, "the call to the testimony that nothing is worthy of worship except Allah, alone with no partner."