Idolatry is all manner of seeming holiness and worshipping, let these counterfeit spiritualities shine outwardly as glorious and fair as they may; in a word, all manner of devotion in those that we would serve God without Christ the Mediator, his Word and command.
In popedom it was held a work of the greatest sanctity for the monks to sit in their cells and meditate of God, [solitude] and of his wonderful works; to be kindled with zeal, kneeling on their knees, praying, and having their imaginary contemplations of celestial objects [meditation], with such supposed devotion, that they wept for joy.
In these their conceits, they banished all desires and thoughts of women, and what else is temporal and evanescent. They seemed to meditate only of God, and of his wonderful works.
Yet all these seeming holy actions of devotion, which the wit and wisdom of man holds to be angelical sanctity, are nothing else but works of the flesh. All manner of religion, where people serve God without his Word and command, is simply idolatry, and the more holy and spiritual such a religion seems, the more hurtful and venomous it is; for it leads people away from the faith of Christ, and makes them rely and depend upon their own strength, works, and righteousness.
Martin Luther from his Table Talk