"The eye is a light for the body. If your eyes are good, your whole body will be full of light. But if your eyes are evil, your whole body will be full of darkness. And if the only light you have is really darkness, then you have the worst darkness.” Matthew 6: 22-23 NCV
Singleness of the eye and heart means that you set your attention upon God for the purpose of doing His will. This is a fundamental principle of spiritual vision.
The opposite of single vision is of course double vision.
Double vision occurs when our two eyes do not look in the same direction. One eye is looking at an object and the other eye is looking at something else. The result is that you see a single object as two objects. This can also happen with our spiritual vision.
Instead of having our whole attention focused on God, we have one eye on God and another eye on the world. Our loyalty is divided between God and someone or something else. That is spiritual double vision.
In the context of Matt. 6:19-24, there is another thing called mammon, i.e., the riches of the world. The loss of vision is caused by the selfish ambition to lay up treasure for ourselves on earth.
It plunges us into moral darkness. “But if your eye is bad (if your eye is not single), your whole body will be full of darkness”. Jesus deliberately uses the word ‘darkness’ because it is a picture of blindness in the Bible.
You find that in Psalm 69:23 for example: “Let their eyes be darkened, so that they do not see”. In Acts 13:11 we read that a magician was trying to oppose Paul. Paul had to deal severely with him and blindness came upon that magician: “A darkness fell upon him (that is, he became blind), and he went about seeking those who would lead him by the hand”.
When a man looks with a double interest upon both earthly and heavenly treasure, it makes him double minded. In fact, he is called a double-minded man in James 1:8. He is unstable in his commitment. This double-minded person, the man with spiritual double vision is the kind of person who gets nowhere in his spiritual life. His whole life is in darkness.
Jesus warns us that we have to make a choice. It is a choice that has to be made between serving God and serving material things. Every man without exception has committed his life to one of two treasures: mammon or God.
His heart and his eyes are focused upon earthly things or upon heavenly things. God does not accept a person who cannot make up his mind and ends up giving his attention to both.
You have to make a definite choice. You either serve God or you serve material things. But you cannot serve God and mammon at the same time. When two masters call upon a man at the same time, that person has to choose which master to favor. In cleaving to one, he automatically rejects the other.
You cannot have one foot in the kingdom of God and the other in the world. God does not accept the divided man.
James says, “Let not that man (the double-minded person) expect that he will receive anything from the Lord” (James 1:7).
The only kind of Christian acceptable to God is the person who is totally committed to God, whose eye is single.