I suggest you read the following article:
http://quransmessage.com/articles/prophet%20muhammad%20as%20a%20mercy%20to%20creation%20FM3.htm
Some points to be noted:
(1) THE PROPHET IS ONLY A MERCY FOR THOSE THAT BELIEVE
(2) THE CAPACITY OF THE PROPHET AS A MERCY ONLY EXTENDS AS A 'MESSENGER' TO WARN HIS OWN PEOPLE
http://quransmessage.com/articles/prophet%20muhammad%20as%20a%20mercy%20to%20creation%20FM3.htm
Some points to be noted:
(1) THE PROPHET IS ONLY A MERCY FOR THOSE THAT BELIEVE
(2) THE CAPACITY OF THE PROPHET AS A MERCY ONLY EXTENDS AS A 'MESSENGER' TO WARN HIS OWN PEOPLE