Jesus makes a fascinating comment about the world. The sinfulness of the heart can be hidden well. It sometimes takes much light to expose the real desires of the heart. The religious leaders of Jesus day gave overwhelming evidence of their goodness until Jesus walked into their lives. Jesus exposes their hearts by his very presence. Light always does that!
John 15:24 says, "If I had not done among them the works which no one else did, they would not have sin; but now they have both seen and hated Me and My Father as well." The works of Jesus exposed their hearts and cause them to act according to their hearts. They appeared religious, but the works of Jesus revealed the deep hatred that they had hidden well from others.
Light always reveals the hidden secrets of the heart. No one is hidden from the eyes of our Lord. He knows our hearts. The Lord will walk us along paths that will evenutally reveal the motives of our hearts.