Why Christian see it as evil to open your third eye?
I would say it is the unknowing and that they can not answer for the things they do not know or teach.
I rather think that falls somewhat short of the actuality.
'The third eye' is rooted in Asiatic Traditions, but the Abrahamics have 'the eye of the heart'.
It's to do with how the person is conceived. One the one hand, the mind encompasses all. On the other, the heart. Neither is wrong, both are pre-eminent in their own domains.
To repeat, in answer to Juice's original question, one will always find fundamentalists of every ilk that reject out of hand anything they consider alien.
A more cautious and mindful approach will soon come upon healthy and informative dialogue of understanding without rancour or the desire to win.
As St Paul said: "I ask that the eyes of your heart may be enlightened." (Ephesians 1:18)
The heart loves, and love, as we know, 'surpasses all reason', in the same way that, in all Traditions, the Ultimate, the Absolute, in whatever form it is spoken, transcends the mind, transcends the intellect, as, for example, in speaking of the Tao, the sacred scribe says: 'The Tao that can be spoken is not the true Tao.'
St Augustine said: "Believe in order to understand", not the other way around. The rational mind says "I will believe when I finally understand", but of course then you’ll never understand. It's a bit like saying "I'll begin the journey when I get there." This is what Christ meant when He said 'suffer the little children' (Matthew 19:14). Children believe with the heart. As Christ said, "Blessed are the clean of heart: for they shall see God" (Matthew 5:8).
In the abrahamic Traditions, the heart (Heb: Leb, Gk: kardia, Lt: cor) stands for the inner, essential person. It refers to, and encompasses, the will, the mind, the consciousness, one's emotions and one's understanding. It also refers to moral character and determination. It is the place of knowledge, memory and reflection. (Such processes belong to brain function, but in Abrahamic anthropology, the brain belongs to the heart.)
In all traditions, the heart is the 'core' of the person, or the being.
In the West, the Lakota Sioux: Black Elk said: "We can all see in the day, and this seeing is sacred for it represents the sight of that real world which we may have through the eye of the heart."
In the East:
Su Dongpo prided himself on his wit and liked to debate Master Foyin. One day, over tea, he challenged the Master. "Foyin, people think you are an enlightened monk, but to me you just look like a big, stinking pile of worthless dung sitting on your pillow all day long."
Master Foyin placed his hands in prayer position. “My dear Dongpo, but to me you look like a Buddha.”
Su Dongpo grinned and bid Master Foyin farewell. When he got home, he said to his sister: "Today I outsmarted Master Foyin," and told her of their exchange.
"Oh no, brother! I’m sorry to tell you this, but you lost badly," she said. "Don’t you realize that the world mirrors the heart? Master Foyin sees you as a Buddha because he is a Buddha. You see him as a pile of dung. What does that make you?"