The Bronze Serp Is a Type of Christ

In Numbers 21, God had Moses build bronze serpent that they could look to and be healed of snakebite. By the time of 2 Kings 18, the Israelites had begun to regard the bronze serpent as a deity. They've turned it into an idol and are committing idolatry in the proper sense. Abuses non-tolate usum does not annul the proper use of something.