Remember Islam is the final revelation 
God, Abraham, David, Moses, Jesus or Muhammed did not preach Judaism, Christianity or Islam

They all preached a heavenly path based on humanity

Sufism existed before Islam, during Islam and after Islam 

Its not necessary for a Sufi to be a Muslim 

A Sufi can exist anywhere in any form 

It has nothing to do with Islam in particular 

Sufism can exist without Islam 

But 
Islam cannot exist without Sufism 

because 
Sufism is the spiritual doctrine of Islam