Pineal Gland 101

The pineal gland has been one of the main subjects of enlightenment for numerous spiritual paths. Although each gland of the human body has been associated with a particular chakra, and thus requires awakening/knowing, the pineal gland seems to be the ultimate goal of personal knowing, or gnosis, through its association with the third eye.

The human body, according to the Tantricas, is the best medium for realizing the truth. This body is not merely a thing in the universe, it is an epitome of the universe, a microcosm in relation to the macrocosm. - Swami Madhavananda, Great Women of India; cited by Elizabeth U. Harding, Kali, the Black Goddess of Dakshineswar.

Alex Grey

As Dr. Svoboda describes in his book Aghora, the Mother/Feminine is the first guru. When you are born, your particular biological mother is evident found through the intimate connection of the umbilical cord. However, your biological father is not so evident, where you require your mother to show you your biological father. Spiritually, the spiritual Mother, Kundalini Shakti, rests at the bottom of the spinal cord. It is through knowing Her that we awaken Her energy, as She brings us to our Father-Higher Self-Shiva seated in the third eye. At the third eye, you have the reunion of the Mother-Father, Feminine-Masculine, essentially a unification of duality, which is a common goal among the mystics. 

The light of the body is the eye: if therefore thine eye be single, thy whole body shall be filled with light.  -Matthew 6:22

Vatican

The pineal gland has been known among the ancients and is often openly used in numerous traditions where the masses remain clueless as to its esoteric purpose. The gland was named "pineal" due to its resemblance of a "pine" cone, a shape that can be found in religious settings, as well as home decor. The Catholic Church has been known to openly use such symbolism at the Vatican, as well as on the staff often carried by the Pope.

Much of western tradition is derived from Greco-Roman-Egyptian collaborations, hence, the reasoning for the Catholic churches use. The caduceus is a perfect example with an appearance resembling the chakra system with the serpents representing the energetic nadis, and a meeting point of the nadis at the third eye. The caduceus has been associated with Hermes(Greek), Thoth(Egyptian), and Mercury (Roman), all deities associated with esoteric knowledge. Moreover, new age interpretation have associated the Egyptian Eye of Horus as symbolic of the pineal gland.

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..when the Spirit Fire is lifted up through the thirty-three degrees, or segments of the spinal column, and enters into the domed chamber of the human skull, it finally passes into the pituitary body (Isis), where it invokes Ra (the pineal gland) and demands the Sacred Name. 
-Manly P. Hall, Secret Teachings of All Ages

Eye of Horus and the Third Eye

Although the West has maintained the third eye through subtle symbolism, the East has placed the third eye within the purview of all spiritual seekers. In the Buddhist tradition, the head of the seated Siddharta Guatama is often in a rugged matted shape argued to resemble the pineal gland, symbolizing an awakened master. Moreover, Eastern traditions have used numerous symbolism to stress the importance of the third eye. Often seen as a fashion statement, the bindi is widely known and appropriately dresses the third eye. Esoterically marked either in red to symbolize blood (feminine, creation, energy) or its male counterpart in white, the bindi is perfect representation of the single point of creation and unification of duality.   

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As described in David Wilcock's The Sourcefield Investigations, the pineal gland was of interest to numerous philosophers including Rene Descartes who believed the pineal gland was the junction point between the body and soul. Wilcock further investigates the pineal gland and finds hundreds of micro-crystals mainly composed of calcite floating within the gland. The crystals are described as "piezoelectric", expanding and contracting  in the presence of electromagnetic fields. Such crystals can be used to tune in to radio stations without the use of electricity and are capable of giving off different colored light. Furthermore, the pineal gland produces melatonin, serotonin, and possibly the chemically similar psychoactive chemical DMT (the active agent in Ayahuasca). 

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Therefore, in its association with crystals, light, and psychoactive compounds, the pineal gland may be our ticket to something that is beyond our body in what we typically witness with our dual eyes. The ancients knew the importance of this gland and kept the tradition alive through subtle symbolism for a later generation to reawaken. Hence, it may be one of the key goals for any spiritual aspirant to unite the Female and Male nadis within the third eye to experience the oneness described as Shiva-Shakti.