The first commandment forbids the sin of idolatry. For this you know with certainty, that no immoral or impure person or covetous man, who is an idolater, has an inheritance in the kingdom of Christ and God. The worship that is due to God is twofold, external and internal. The external has to do with the body. When a man bows to something, it is an act of worship.
Internal worship consists of the acts of the soul.
It is spiritual in nature. When the mind is taken up with an object, when the affections are set up on it, this is soul worship, and it is due only to the true and living God. These two kinds of worship, external and internal, mean that there are also two kinds of idolatry. There is open idolatry and secret idolatry. Open idolatry is bowing before anything besides the true and living God.