The third proof for the RPW comes from the Second Commandment’s prohibition against idols. It was James Ussher who influenced the Westminster divines to settle that the Second Commandment taught the RPW. His unique contribution to the Assembly shows itself brilliantly in his catechism, “What is the scope and meaning of this commandment [the Second]?” The answer, “to bind all men to that solemn Form of Religious Worship which God himself is in his Word precribeth: that we serve him not according to our fancies, but according to his own will (Deut 12:32).” This answer is followed by the question, “What is generally forbidden herein?” And its answer:
Every Form of Worship, though of the true God (Deut 12:31) contrary to, or diverse from the prescript of God’s Word, (Mat 15:9) called by the Apostle Will-worship, (Col 2:23) together with all corruption in the true worship of God, (2 Kings 16:10) and all lust and inclination of the heart unto superstitious pomp and rites in the service of God.
“Will-worship” from Colossians 2:23 became the term that marked worship methods not found in Scripture. It was worship that man had willed but not God. Ussher argued that it was idolatry. The Puritans jumped on this term and charged it to the practice, whereby human rites and ceremonies were added to the worship service. The Second Commandment forbids man-made practices.
 James Ussher, A Body of Divinity: The Sum and Substance of Christian Religion, (Solid Ground Christian Books, 2007-06-12), 199.