Idolatry is often subtle. The subtlety comes because we tend to make idols of good things. We can fixate so much on a particular good thing that it becomes an ultimate thing. Anytime we think that we cannot be happy or satisfied as a result of not having something, we have made it an object of worship—an idol. In the pursuit of self-referential freedom, we often place ourselves in the bonds of spiritual slavery.
In Galatians 4, the Apostle Paul confronts false teachers in the churches of Galatia who were seeking to enslave the Galatians by teaching that spiritually mature believers trust Christ and keep various aspects of the law, including circumcision. He explains that before they knew God, the Galatians were enslaved in the worship of many so-called gods and all manner of vain idols. Their lives as pagans were involved in all sorts of false worship, sin, and immorality. However, once the Galatians heard and believed the gospel of the free and sovereign grace of God in Christ, they were freed from their slavery to sin and the enslaving pursuit of idols.
But some in the churches of Galatia were beginning to believe the Christ-plus-something message of the false teachers.
This is how Pastor David starts a must read post on the dangers of moral idolatry. You can read the whole thing here.