The light of the body is the eye: therefore when thine eye is single, thy whole body also is full of light; but when thine eye is evil, thy body also is full of darkness. Take heed therefore that the light which is in thee be not darkness. If thy whole body therefore be full of light, having no part dark, the whole shall be full of light, as when the bright shining of a candle doth give thee light.
The importance, position, and power of the eye in the body are very precious. When the tempter came to Eve in the Garden of Eden, he fully exploited the importance of the eye, when having her eyes focus on the forbidden tree, he was able to have access to her soul.
The same strategy of Satan is fully at work today. The sooner a believer understands this and asks the Lord for an eye salve (Rev. 3:18) the whole body will be full of light.