I don’t. One can believe that there is one God but Muhammad in not his Prophet or maybe one believes his prophecy was filtered through the prism of his own personality, circumstances and environment, making it a prophetic message, like all prophetic messages in my opinion, that have to be placed in context. To me the distinction between the Medina and the Mecca suras highlight this point. Or, maybe you fully stand behind the Shahada in your heart without compromise but external situations you relate to consciously and possibly unconsciously prevent you from declaring it.