Gladness is the token of the truth and the worth of obedience, showing whether I have pleasure in the will of God. In the Scriptures light and gladness are frequently connected with each other. It is so in nature. The joyful light of the morning awakens the birds to their song and gladdens the watchers who in the darkness have longed for the day. When darkness comes over the soul, it is always through one of two things, through sin or through unbelief.
Sin is darkness, and makes dark.
And unbelief also makes dark, for it turns us from Him, who alone is the light. It is sin, the turning from behind Jesus to our own way, that makes dark. But at the moment we confess sin, and have it cleansed in the blood, we are again in the light. Or it is unbelief that makes dark. As soon as we look to Jesus, to the fulness, to the perfect provision for our needs that is in Him, all is light.