Thank you.
When we get the regulative principal from the second commandment how do we get the categories of elements, forms, rubrics, and circumstances? And how do we get the various ways I which they are regulated by the principal?
The key passages used in deriving the RPW are Deuteronomy 4:2, 12:32, Exodus 20:4-6, Leviticus 10:1-2, Matthew 15:7-9, and John 4:23-24; 1 Corinthians 11:23–26 shows Paul adhering to a direct command. Acts 2:42 shows the church adhering to specific parts of worship (apostles' doctrine, fellowship, breaking of bread, prayer). The Westminster Standards don't speak in multiple categories (forms, rubrics). Think in terms of the worship which is commanded of us being made up of parts (the Westminster divines' term), which must be found by warrant from Scripture. "The reading of the Scriptures with godly fear; the sound preaching, and conscionable hearing of the Word, in obedience unto God with understanding, faith, and reverence; singing of psalms with grace in the heart; as, also, the due administration and worthy receiving of the sacraments instituted by Christ; are all parts of the ordinary religious worship of God", etc. WCF 21.5. In order to do these parts you get the things attending them, the circumstances. To worship, we need a time, a place, lighting to see, if providing seating what kind, etc. Some of these circumstances are commanded in Scripture as well. For the day of public worship, it is the Lord's Day; for the time, it is not commanded and so must be determined by the church, which is governed by wisdom and prudence, avoiding offense, etc. (if free, 5am worship is not wise; if under persecution, perhaps it is). Similarly, the Lord's Supper is commanded as a part of worship; some circumstances are commanded, must be bread and wine, some are not, how frequently the supper is to be observed. "there are some circumstances concerning the worship of God, and the government of the Church, common to human actions and societies, which are to be ordered by the light of nature and Christian prudence, according to the general rules of the Word, which are always to be observed." WCF 1.6. "The communion, or supper of the Lord, is frequently to be celebrated; but how often, may be considered and determined by the ministers, and other church-governors of each congregation, as they shall find most convenient for the comfort and edification of the people committed to their charge. And, when it shall be administered, we judge it convenient to be done after the morning sermon." DfPW.