The second commandment expressly forbids the worship of graven images. Our worship must transcend the creation and reach to the creator. The Israelites were mixing pagan idolatry with what they considered to be YAHWEH worship. Like in math, a positive and a negative don’t make a positive. Despite their apparent sincerity, these people were employing pagan forms of worships in an attempt to worship God. This was idolatry.
