Adam Clarke on Leviticus 26:1

Ye shall make you no idols - See note on Exo 20:4, and see the note on Gen 28:18-19 (note), concerning consecrated stones. Not only idolatry in general is forbidden here, but also the superstitious use of innocent and lawful things. Probably the stones or pillars which were first set up, and anointed by holy men in commemoration of signal interposition of God in their behalf, were afterward abused to idolatrous and superstitious purposes, and therefore prohibited. This we know was the case with the brazen serpent, 2Kgs 18:4.