Matthew Henry Commentary:

“We have here the dignity of the soul, the great soul of man, that light which lighteth every man.

1. It is a divine light; it is the candle of the Lord, a candle of his lighting, for it is the inspiration of the Almighty that *gives us understanding.*He forms the spirit of man within him.  It is after the image of God that man is created in knowledge. Conscience, that noble faculty, is God's deputy in the soul; it is a candle not only lighted by him, but lighted for him. The Father of spirits is therefore called the Father of lights.

2. It is a discovering light. By the help of reason we come to know men, to judge of their characters, and dive into their designs; by the help of conscience we come to know ourselves. The spirit of a man has a self-consciousness (1 Cor 2 11); it searches into the dispositions and affections of the soul, praises what is good, condemns what is otherwise, and judges of the thoughts and intents of the heart. This is the office, this the power, of conscience, which we are therefore concerned to get rightly informed and to keep void of offence.