Remember Dheenul Islam is the final revelation 

God, Abraham, David, Buddha, Moses, Jesus or Muhammed did not preach different religions such as Judaism, Christianity, Buddhism or Islam by name

They all preached a heavenly path based on

HUMANITY and CONSCIENTIOUSNESS 

Followers of that path later changed their path into religions 

However 

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 Dheenul Islam 

which is called the

Dharmaya (Deen)