Regulative Principle of Worship

"The acceptable way of worshipping the true God is instituted by Himself, and so limited by His own revealed will, that He may not be worshipped according to the imaginations and devices of men, or the suggestions of Satan, under any visible representation, or any other way not prescribed in the holy Scripture." - The Westminster Confession. Ch 21.1

A timely discussion of the Reformed "Regulative Principle of Worship" beginning around minute 14:

 http://reformedforum.org/podcasts/ctc236/