The souls of the righteous become heavenly light, the Lord Himself told the apostles, when He said, Ye are the light of the world.. You are the light of the world. A city set on a hill cannot be hidden. Matthew 5:14
He first wrought them into light, and ordained that through them the world should be enlightened. Neither do men light a lamp, He says, and put it under the bushel, but on the lampstand, and it giveth light to all that are in the house.
Let your light so shine before men. In other words, Hide not the gift which ye (you) have received from Me, but give to all that are minded to receive it.
Again, The light of the body is the eye ; if thine eye be full of light, thy whole body is enlightened, but if thine eye be evil, thy whole body is dark. If therefore the light that is in thee be darkness, how great is that darkness?
As the eyes are the light of the body, and, so long as the eyes are well, the whole body is enlightened, but, if any accident befalls them and they are darkened, the whole body is in darkness, so the apostles were set to be the eyes and light of the whole world. The Lord therefore charged them by this saying, If ye who are the light 'of the body, stand fast and turn not aside, behold, the whole body of the world is enlightened ; but if ye who are the light are darkened, how great is that darkness, which is nothing less than the world. So the apostles, being themselves light, administered light to those, who believed, enlightening their hearts with that heavenly light of the Spirit by which they were themselves enlightened.
Abba Macarius - St. Macarius the Great