The Third Eye, Shiv Netra, Divya Chakshu, Tisra Til, Brahmrendra, or Single Eye must be opened to see the Light of God.
It opens when the sensory currents are withdrawn and gathered up to the seat of the soul at the eye focus behind and between the two eyebrows. Then one experiences the mysteries of the Beyond into which a Master-soul initiates a disciple during his lifetime. He gives a first-hand inner experience of conscious contact with the holy Naam -the Divine Light and the holy Sound-Current (Holy Ghost), as the lowest expression of the Divinity within. The light of the body is the eye, if therefore thine eye be single, thy whole body shall be full of light.

Sant Kirpal Singh

The problem is that nobody really knows where the Single Eye is

The problem is, that nowadays all people say: “Here is the seat of the soul, meditate over here!” – between and behind the two eye-brows. But it is a big problem that nobody really knows where the seat of the soul or the Single Eye is. What happens at the time of death? Our attention, the outer expression of the soul or consciousness, withdraws from the lower extremities, and transcending the throat ganglion, it goes to the root of the two eye-brows. From there it takes upward-trend, then it takes inward-trend, and where the eyes are crossing, there is the seat of the soul. To say, “Here is the seat of the soul” is easy, but to reach there and to locate it there is very difficult, because just very close to the seat of the soul, a little bit below, there is the seat of the mind and below there is the seat of the intellect.

What do most people do when they meditate? First, they have to overcome the intellect. Maybe they can overcome it, but they cannot overcome the mind. When we create a wish, our mind at once identifies with it. Mind in this physical body is astral.

Everyone has wishes upon wishes, and our mind identifies with them. So when we create a wish to see something, at once our mind identifies with it and will give us the light over there. Mind will show the light at once, whereas the experience taught by Christ and all the competent Masters starts from the darkness and then leads to the holy light.

Dr. Harbhajan Singh