In what consist to open the third eye in the bible
Jesus makes many references to the eye of the mind or third eye:
"Your eye is the lamp of your body. When your eyes are healthy, your whole body also is full of light. But when they are unhealthy, your body also is full of darkness". Luke 11:34
When we talk about the third eye we talk about all the phenomena related to the reading of thoughts. In other words, we talk about "reading thoughts", not about "hearing voices". The first phenomenon is related with the integration of the mind, the second to division (disorders related to schizophrenia). The third eye is what gives rise to the concept of the Oracle.
When we speak of the third eye we speak about all the communication with the plane of wisdom, this communication may consist of:
1) Do divine justice, and discern between good and evil, detecting deception, like Jesus and the prostitute: "Teacher this woman was caught in the act of the adultery. In the law Moses commanded us to stone such women. Now what do you say? They were using this question as a trap, in order to have a basis for accusing him. But Jesus bent down and started to write on the ground with his finger. When they kept on questioning him, he straightened up and said to them, "Let any one of you who is without sin be the first to throw a stone to her". John 8:1-7
2) Guess the future, along with God's opinion about a matter, question or doubt, such as Joseph and Pharaoh in the book of Genesis: "Pharaoh said to Joseph, "I had a dream, and no one can interpret it. But I have heard it said of you that when you hear a dream you can interpret it". "I cannot do it", Joseph replied to Pharaoh, "but God will give Pharaoh the answer he desires". Then the Pharaoh said to Joseph, "In my dream I was standing on the bank of the Nile, when out of the river there came up seven cows, fat and sleek, and they grazed among the reeds. After them, seven other cows came up - scrawny and very ugly and lean. I had never seen such ugly cows in all the land of Egypt. The lean, ugly cows ate up the seven fat cows that came up first. But even after they ate them, no one could tell that they had done so; they looked just as ugly as before. Then I woke up. In my dream I saw seven heads of grain, full and good, growing on a single stalk. After them, seven other heads sprouted - withered and thin and scorched by the east wind. The thin heads of grain swallowed up the seven good heads. I told these to the magicians, but none of them could explain it to me. Then Joseph said to Pharaoh, "The dreams of Pharaoh are one and the same. God has revealed to Pharaoh what he is about to do". Book of Genesis 41: 15-44.
3) Know hidden things like the three wise men and King Herod: Then Herod called the Magi secretly and found out from them the exact time the star had appreared. He sent them to Bethlehem and said, Go and search carefully for the child. As soon as you find him, report to me, so that I too may go and worship him. After they had heard the King, they went on their way, and the star they had seen when it rose went ahead of them until it stopped over the place where the child was...And having been warned in a dream not to go back to Herod, they returned to their country in another route. Matthew 2:7-12.
These are the most important functions of the third eye, and their origin is in the same knowledge of God: "This is eternal life, that they may know you the only true God", John 17:3. The third eye is the result of the exercise of contemplative activity, of the meditation focused in the attributes and perfections of God. For Thomas Aquinas, contemplative activity is the same vision of God, thence the bliss: "Blessed are the pure in heart, for they will see God," Matthew 5:8.
For Aristotle, contemplation is related to wisdom and this is the most perfect activity, thence the greatest degree of happiness. In the Bible the third eye can only be opened by those who live the gift of godliness or Piety. Moreover, it is a fruit directly related with Piety: constancy, firmness, devotion, equanimity, stability. King Solomon who speaks with God in dreams (1 Kings 3: 5-12), the apostles who receive the holy spirit at Pentecost (Acts of the Apostles 2), are phenomena related with the opening of the third eye.