Those who actualize the truth say--[concerning their understanding of (Qur'an 4:135) ya ayyuha alladhina amanu (O you who have believed) that it is as if the Qur'an is stating]--"O you who have affirmed faith"  on the basis of [textual or intellectual] proof (burhan), "Affirm faith by way of unveiling and direct vision."  

Or [another way of putting it is] " 'O you who have affirmed faith' by way of assent (tasdiq), affirm faith by way of actualization (tahqiq)."  

And it was narrated that Hazrat Qutb al-aqtab Khwajah Baha' al-Din Naqshband--may his sirr be sanctified--said, " 'O you who have affirmed faith' alludes to the fact that at every blink of the eye one must negate this human existence and affirm the Necessary Being--may His remembrance be glorified.' " 

"Your existence is a sin to which no [other] sin compares." 

It was narrated that the Hazrat, the Master of the Sufis,  Junayd--may Allah sanctify his spirit-- said concerning the meaning of "O you who have affirmed faith," "It has been fifty years that I have been involved in my affirming of faith and in my renewing of faith, and I am still involved in that." 

Mathnawi (of Mawlana Rumi): 

It is utter sin to take a breath without God 
To become preoccupied with oneself is to disbelieve in the path. 

Each breath the thought of existence pulls you 
toward the realm of darkness of self-worship. 
  
Selfhood is disbelief, negate your self quickly, 
since in truth only God exists. 

(End of passage from Mavaheb-i aliya, Tafsir-e Husayni; 
translated by Dr. Alan Godlas, University of Georgia ©1998) 
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Scholarly note: The expression I have translated as "to affirm faith" (iman avardan) literally translates as "to bring faith" and is the standard Persian translation for the Arabic "amana" (to believe, to have faith, or to affirm faith).