Source - Light Within Me, Page (1), by Allamah Husayn Tabataba`i, Shaheed Murtada Mutahhari and Ayatullah Ruhullah Khumayni
Although most people are preoccupied with earning their livelihood and pay little attention to spiritual matters, yet every man has an inherent desire to know the absolute truth. When this dormant power comes to surface and is awakened in some people, they gain a number of spiritual perceptions.
Despite the claim of the sophists and the atheists that every truth is an illusion, everybody believes in the existence of one eternal truth. When man with a pure heart and a pure spirit looks at the permanent factuality of the universe and at the same time observes the instability and transience of its various parts, he realizes that this world and its manifestations are a mirror which reflects the existence of one eternal truth. With this realization his joy knows no bounds and he is so elated that in his eyes everything else becomes insignificant and worthless.
This spectacle forms the basis of that impulse of the Gnostics [1] which draws the attention of the godly people to a world beyond perception and cultivates the love of Allah in their hearts. The pull which they feel towards this spectacle makes them forget everything and removes many desires from their hearts. This pull leads man to the worship of the Invisible Being who is more manifest than all that is visible or audible. It is this pull which gave birth to many a religion based on Allah's worship. The real Gnostic is he who worships Allah not, because he hopes for any reward or is afraid of any punishment, but only because he knows Him and loves Him. [2]
It is clear from the above that gnosis is not a religion like other religions. It is to be regarded as the central and the most vital part of all religions. Gnosis is a perfect way of worship, based on love, not on fear or hope. It is a way of understanding the inner facts of religion instead of being contented with its outward and perceptible form. Among the followers of all revealed religions, even among those who believe in idol-worship there are individuals who follow the path of gnosis. The Gnostics are found among the followers of polytheistic religions [3] as well as among the Jews, Christians, Zoroastrians and Muslims.

Notes:
[1] The Islamic esoterics known as Irfan or gnosis is sometimes associated with Tasawwuf or mysticism whose certain rites and rituals are repugnant to Islam. However Shi'aism considers Islamic acts of worship to be sufficient for gaining proximity to Allah.
[2] Imam Ja'far Sadiq has said: "There are three categories of the worshippers: 'Those who worship Allah out of fear; their worship is that of the slaves. Those who worship Allah for the sake of a reward; their worship is that of the wage-earners. Those who worship Allah out of love and earnestness; their worship is that of the freeman. This last is the best form of worship." (Biharul Anwar, vol. V, p. 208).
[3] Here the learned author has in his mind the religions of India and the Far East in which different aspects of divinity are represented by gods and godesses in a mythical and symbolic form