The words used in the Basis of Union to describe our public worship practice are simplicity and purity. Both words express the same general meaning. They state that our public worship practices are unmixed, that is, they conform the Regulative Principle as it set out in chapter one of the Westminster Confession of Faith at section six. If there is a distinction to be made between them, simplicity would include the thought that our public worship is unadorned, and purity would place the emphasis on it being unalloyed, by the traditions of men.
Simplicity and purity of public worship practice is often shortened to purity of worship. The latter form of words is open to being misunderstood. Purity of worship speaks to the way in which things are done. It says nothing about those engaged in the practices. No matter how pure the practice is, the worshiper, in this life, is not pure, but needs both the aid of the Holy Spirit and the advocacy of Christ, our Great High Priest, in coming before God.
We accept the Westminster Directory for the Public Worship of God as a scriptural and suitable guide for the conduct of public worship. It is not our liturgy or form, nor does it provide definitive orders of service. Making allowance for space and time, its guidance informs our practice. It reminds us how we might keep it simple.