Why did the people of Mecca reject the message of Islam? Was it because they rejected the idea that Muhammad (peace be upon him) could be Allah's Messenger or was it for some other reason?
The answer to your question is made clear by Allah in the Qur’ân. Allah relates to us their objection, which was: “Has he made gods (all) into one God! Truly this is a strange thing” [Sûrah Sâd: 5].
Allah had sent his Messenger Muhammad (peace be upon him) with monotheism and asked him to say: “O my people! Worship Allah! You have no other god but Him” [Sûrah Hûd: 50].
If the Prophet (peace be upon him), when he came to them, called them just to believe in his messengership while leaving them on their polytheism, then they would had accepted him as a Prophet and Messenger. But he came to them with a call to worship one God and only one God. Even with their knowledge that the Prophet (peace be upon him) was a man who always spoke the truth and never lied, they rejected his call.
His uncle said so much in one of his poems. he wrote: “If I could save myself from being blamed, I would have joined Muhammad’s religion, as it is the best religion.”
He confirmed the Message of the Prophet (peace be upon him) and his messengership, but he refused to enter in his religion and preferred to die on the religion of his parents and grandparents.
And may peace and blessing be upon our Prophet Muhammad.
Source: Islam Today
-- Al Arabiya Digital