The eye can mean ones perspective, or outlook on life. It can either be worldly, focusing on material possessions and personal image, or it can be spiritual, focusing on God's eternal perspective and his gifts and other blessings so liberally given us.
Lamp can be translated light, which is a symbol of truth. So a healthy eye is one that views life from God's eternal perspective, and the whole body is ones unique set of values, attitudes, beliefs, dreams and ambitions.
The bad eye, however is a perspective that is clouded by worldly cares, possessions, personal image, philosophies, attitudes and beliefs that yield only the darkness of fundamental error. Normally, darkness is simply lack of illumination, but this whole body darkness is even deeper than that. It is the darkness of a double-shielded chamber; first, no light exists within, and second, external light is blocked by the indelible prejudices that form around a life committed to worldliness and naturalism.
Speaking of prejudices, though, worldliness and naturalism are not the only ones that block the light of truth. Religious prejudice from deep-seeded human tradition can block truth at least as effectively as any secular lie. God's truth, quite tragic for those whose bad eye prevents its appreciation, makes no distinction between secular darkness and religious darkness. All who live in darkness, those who actively shun God's light, as well as those who passively miss its obvious existence in creation, will one day stand in judgment, in God's unapproachable light, and hear the fateful words that their choices made inevitable:
"Then he will say to those on his left, 'Depart from me, you cursed, into the eternal fire prepared for the devil and his angels. For I was hungry and you gave me no food, I was thirsty and you gave me no drink, I was a stranger and you did not welcome me, naked and you did not clothe me, sick and in prison and you did not visit me.' Then they also will answer, saying, 'Lord, when did we see you hungry or thirsty or a stranger or naked or sick or in prison, and did not minister to you?' Then he will answer them, saying, 'Truly, I say to you, as you did not do it to one of the least of these, you did not do it to me.' And these will go away into eternal punishment, but the righteous into eternal life." (Mat 25:41-46 ESV)The Judge will not discriminate between varying shades of doctrine, ones faithful adherence to sacred ritual, or ones position within the church hierarchy. Jesus will simply judge the love shown, or not shown, to the least of these.
Some will complain that this final judgment is based on our works, while elsewhere, God's word states that we are not saved by our works, but by our faith alone. Such complainers will be disappointed to learn there is no conflict here at all. Simply put, we cannot have saving faith without first allowing God's light to shine into our fallen souls. First, God's light. Next, God's saving faith. Finally, obedience growing out of God's saving faith. No one can have the last without the first, and neither can one truly have God's all-encompassing light without producing the resultant saving faith and the obedient works that it produces.