ignorance or neglect of His Word and will is idolatry.
They may think about God, Christ, and heavenly things, but they do it after their own fashion and not after the Word of God. They have an idea that their clothing, their mode of living, and their conduct are holy and pleasing to Christ. They not only expect to pacify Christ by the strictness of their life, but also expect to be rewarded by Him for their good deeds. Hence their best "spiritual" thoughts are wicked thoughts. Any worship of God, any religion without Christ is idolatry. In Christ alone is God well pleased.
I have said before that the works of the flesh are manifest. But idolatry puts on such a good front and acts so spiritual that the sham of it is recognized only by true believers.
Martin Lurther from his Commentary on Galatians