Here’s a very nice passage from The Niche of Lights by Al-Ghazali –
The gnostics (see my note below), after having ascended to the heaven of reality, agree that they see nothing in existence save the One, the Real. Some of them possess this state as a cognitive gnosis. Others, however, attain this through a state of tasting. Plurality is totally banished from them, and they become immersed in sheer singularity. Their rational faculties become so satiated that in this state they are, as it were, stunned. No room remains in them for the remembrance of any other than God, nor the remembrance of themselves. Nothing is with them but God. They become intoxicated with such an intoxication that the ruling authority of their rational faculty is overthrown. Hence, one of them says, “I am the Real!” another , “Glory be to me, how great is my station!” and still another, “There is nothing in my robe but God!”
Note: The Arabic word translated here as gnosis is ‘irfan, whose literal translation into English is knowing. In its religious sense, ‘irfan refers to realization or direct spiritual knowledge. The Arabic word for one who has attained ‘irfan is ‘arif, here translated as gnostic. In Islam, these terms are often associated with Sufism.
The title of Al-Ghazali's book refers to a passage (24:35) in the Qur'an, sometimes known as the Parable of Light, which is held in particularly high esteem among all Muslims and especially among Sufis. It reads as follows:
God is the Light of the heaven and the earth.
The parable of His Light is as if there were a niche,
And within it a lamp,
The lamp enclosed in a glass,
The glass as it were a brilliant star,
Lit from a blessed tree,
An olive, neither of the east nor of the west,
Whose oil is well-nigh luminous,
Though fire scarce touched it.
Light upon Light!
God guides whom He will to His Light;
God sets forth parables for men;
And God knows all things.