Matthew 6:19-24
19 Lay not up for yourselves treasures upon earth, where moth and rust doth corrupt, and where thieves break through and steal:
20 But lay up for yourselves treasures in heaven, where neither moth nor rust doth corrupt, and where thieves do not break through nor steal:
21 For where your treasure is, there will your heart be also.
22 The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light.
23 But if thine eye be evil, thy whole body shall be full of darkness. If therefore the light that is in thee be darkness, how great is that darkness!
24 No man can serve two masters: for either he will hate the one, and love the other; or else he will hold to the one, and despise the other. Ye cannot serve God and mammon.
The Grace New Testament Commentary - Matthew 6:22-24
6:22-24 The second benefit is that a good eye promotes spiritual enlightenment throughout the total person. That person knows the true value of things and lives accordingly. A bad (evil) eye is envious, covetous, or grudging. Such an eye leads to distortion, or even worse, loss of spiritual vision so that the person is enveloped in spiritual darkness.
The conflict between light and darkness manifests itself in conflicts of interests and priorities. Which shall have first place, God (heavenly concerns) or mammon (riches and earthly concerns)? A person cannot serve both masters, for one will eventually win out and exercise control over his life.