The best religion, the most fervent devotion without Christ is plain idolatry. It has been considered a holy act when the monks in their cells meditate upon God and His works, and in a religious frenzy kneel down to pray and to weep for joy. Yet Paul calls it simply idolatry. Every religion which worships God in 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 regarded 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.
Luther, Martin, Commentary on the Epistle to the Galatians ( 5:19-20).