It should be noted that Jesus was teaching to a particular society and culture—some 2,000 years ago in another land and in another language. Each culture and society has tangible and common everyday occurrences and understandings that might be used to communicate a deep subject matter.
Here Jesus was teaching about the deepest subject matter of all: He was discussing elements of a world transcendental to this temporary physical world. Because this transcendental world is unseen by the physical eyes or physical mind, it is difficult to describe without using metaphorical language.
This of course creates a problem of understanding for those who either do not relate to the metaphors being used or are not otherwise versed in the subject matter. In Jesus' times, people used lamps with fire for light. A lamp could be of different shapes and sizes, and a lamp could have a good light - allowing a person to see quite a bit - or a poor light - allowing for little vision.
Jesus is comparing the "eye" to the condition of ones heart and the "light" to ones spiritual condition - ones relationship with the Supreme Being.
A person who is soft hearted and full of humility will be able to receive the teachings of God's representative and thus will be able to re-develop their relationship with the Supreme Being. That will leave their "light" - the condition of their relationship with the Supreme Being - in a spiritually-good state - increasingly joyful and full of love and compassion.
But if ones heart ("eye") is full of pride, greed and self-centeredness, then there would be little ability to receive Jesus' teachings. And their "light" within - their spiritual condition - will be in darkness.
What does he mean by "darkness"?
"Darkness" is that state where our self-centeredness and our pride prevent us from connecting with our relationship with the Supreme Being. This is "darkness" because we are each intimately related with the Supreme Being. Yes, we were created by Him. But we were created in order to exchange a unique loving service relationship with the Supreme Being.
This is our identity. This is who we are. We are not these physical bodies. We are each spirit-persons who belong with Him in the spiritual realm.
But because love requires freedom, the Supreme Being also created us with the freedom to love Him or not. We can reject Him if we want, in other words.
Those who rejected their relationship with the Supreme Being were sent to the physical world and given physical bodies to act out our desires for independence and self-gratification.
But in order to maintain our freedom, the Supreme Being also created the physical world in such a way that it completely covers up our spiritual identity and our relationship with Him. It allows us to completely escape Him, and even forget Him and deny His existence.
Such a state is the state of "darkness" that Jesus is referring to in this metaphorical statement. He is talking about a person who wants nothing to do with the Supreme Being. They don't want to love Him. They don't want to serve Him. They want to be Him.
This is why so many of us are struggling to be the top-dog. We want to be the 'champion of the world.' We want to be 'the greatest.' We want to be 'the hero.'
These are all the Supreme Being's positions. Yes, we became envious of the Supreme Being. That is why we are away from Him.
When Jesus says “how great that darkness,” he points out that when the heart is blackened by greed and envy, a great downward spiral draws the self deeper and deeper into self-centeredness - leading to acts of hatred, anger and violence.
But we can turn things around quite quickly and immediately. We can transcend this darkness by simply turning to the Supreme Being, and praying to Him - Our Friend and Eternal Savior - to rescue us. For this reason, God is often referred to as The Most Compassionate, because we can approach Him in distress, and He will come to our rescue. Because He is always there for us. This is called unconditional love.
Such a request made humbly will immediately soften our hearts and allow our innate relationship with the Supreme Being to begin to bring light into our lives.
(For a translation of Jesus' statements from the Gospel of Matthew without institutional sectarian influence, see the Gospels of Jesus - translated from the original Greek texts.)