When we give witness of the Messenger as in Shahada, ‘I bear Witness that Mohammad (pbuh) is a messenger of Allah’, do we give witness of the Messenger of Allah as Allah described him in Al-Quran or are we giving witness of a historical figure of Mohammad (pbuh) from sources other than Quran?, there are no sources as reliable as Quran and also Quran warns in 23:69, that do they not recognize their Messenger?. If we are holding on to a personality of the Messenger which is different from what is described in Al-Quran, then we are not witnessing for the Messenger of Allah but to a different personality of Messenger which is a historical figure. In order to believe in the correct personality of Mohammad PBUH, we need to believe that personality of Messenger which is depicted in Quran. Some people are so much into the other personality of the Messenger of history, that when Ayas of Quran are presented to them about true personality of Messenger Pbuh, they just refuse to believe in those Ayas.
The above link to this post explores Ayas of Quran for the personality of the Messenger in Quran.