As a former charismatic, I recently had the opportunity to meet up with one of my former pastors to catch up and share how we had been growing in our faith, and we had a great discussion surrounding worship, why I only sing Psalms, why instruments were a shadow of what was to come and are thus fulfilled, etc. He finds this position intriguing, and wants to learn more. He has never heard of the Regulative Principle of Worship before. I know a lot of good resources focusing on how the RPW is to be applied, but I want to give him something that lays out the general, biblical and historical foundations for the principle. Any recommendations? I am open to articles from non-EP, non-acappella perspectives, as long as they do not explicitly teach this (I am not going to send him an article that calls instruments a circumstance for example, but I would send him something that remained silent on the matter). Thanks!