Invoking the letters of God's Name without presence of mind is invocation of the tongue; invoking with presence of mind is invocation of the heart; and invoking with an absence of self-awareness because of absorption in the Invoked is the invocation of the Self - this is the hidden invocation!
Source: The Key To Salvation: A Sufi Manual of Invocation, 1996. p. 51
Let your invocation be the all-embracing Name, which is Allah, Allah, Allah, or if you so wish, Huwa, Huwa, Huwa; and do not violate this remembrance. Be careful lest your tongue pronounce it while other-than-He is in your heart. Let your heart be the one who utters, and your ear the one who is attentive to this invocation until the 'speaker' emanates from your Self (sirr). When you feel the emergence of the Speaker within you through the invocation, do not abandon the spiritual condition wherein you find yourself.
Source: The Key To Salvation: A Sufi Manual of Invocation, 1996. p. 108
The reality of the invocation is when the Invoked takes possession of the heart, and He is One. Separation and multiplicity exist before that for as long as the invoker is in the station of invoking with the tongue or with the heart.
Source: The Key To Salvation: A Sufi Manual of Invocation, 1996. p. 117
God Most High has said, "Is the reward of virtue aught save virtue?" . . . Know, O man, that the covenant of servanthood is incumbent upon you, and that the covenant of Lordship is incumbent upon His magnanimity, as He Most High has said, ". . . and fulfill your covenant, I shall fulfill My covenant."
Source: The Key To Salvation: A Sufi Manual of Invocation, 1996. p. 159
Imam al-Ghazali: God is much too great for the senses to reach Him or for reason and logic to plumb the depths of His Majesty. Indeed, He is much too great for anyone but Himself to plumb the depths of His Majesty or for anyone but Himself to know Him. Verily, no one knows God but God. The highest degree of gnosis that His servants can attain is the realization that true gnostic knowledge of Him is impossible for them. Furthermore, no one can know that in its totality except a Prophet of a righteous saint (siddiq). As for the Prophet, he has clearly expressed this by saying, "I cannot enumerate the ways of praising Thee; Thou art as Thou has praised Thyself." As for the righteous saint, he says, "The incapacity to attain realization is a realization."
Source: The Key To Salvation: A Sufi Manual of Invocation, 1996. p. 113
Among their attributes: renouncing both worlds from their hearts, and preferring their brethren among God's creatures to what they possess; relying upon God Most High in all their affairs; being content with everything that happens to them which the ego detests; and bearing patiently sufferings and separation from one's homeland. They dissociate themselves from creatures without believing that there is evil in them, but rather out of preference for the Creator over creatures. They sever relationships, surmount obstacles, and strive in fulfilling the needs of people after emptying themselves: for whosoever strives to do that before emptying himself is actually seeking leadership and a good reputation.
Source: The Key To Salvation: A Sufi Manual of Invocation, 1996. p. 128
Know that the science of unveiling has no end to it, for it consists in the journey of the intellect in the stations of Majesty, Beauty, Sublimeness, Grandeur, and Holiness. . . . He to whom the mysteries of La ilaha illa'llah are revealed draws near to God, and his worship of God becomes sincere. He does not turn to anyone but to Him, nor does he have hope in or fear other than Him, nor does he see harm or benefit except as coming from Him. He abandons whosoever is not He and rids himself of inward and outward associationism (shirk).
Source: The Key To Salvation: A Sufi Manual of Invocation, 1996. p. 140