In my humble opinion, the simplest and most accurate definition of a Muslim today is one who believes in the exalted Muhammad as the final prophet/messenger of God.
Following the anti-Qadiyani riots in 1953, THE JUSTICE MUNEER COMMITTEE assembled top 31 'Ulema' of Pakistan asking them to define a Muslim. While 30 of them requested one to six months time to contemplate, only the Barelwi Chief Mullah, Shah Ahmad Noorani of Karachi, came up with his 100 page definition in one week. He invited scathing 'fatwas' from all others. Eleven 'Ulema' each came up with their own conflicting views in two to five months issuing fatwas against each other, and the rest kept mumb.
The Bench comprising Justices Muneer and Kayani wrote in their judgment: "Now we are unsure of ourselves if we are Muslims or not. We request the nation to spare our lives."