They become idols when people idolize them.
LOL. Well, I’ll not disagree with that. But what does it mean to “idolize” something? In pagan theology, the idol had power because the spirit of the demon had entered it. It became almost a living thing. No Christian that I ever heard of thought that the spirit of the deceased saint entered into the relic. If the relic had any “power” at all, it was because Almighty God chose to perform a miracle through the intercession of the saint involved. Even the very earliest Christians believed that which is why you have innumerable stories in antiquity of Christians retrieving the bodies of saints and pagans attempting to destroy the body to deny the Christians their relics.
Like how Protestants idolize their personal bibles?