The Form of Sound Words
Christ in the Scriptures has instituted a government for his church. There are to be governing assemblies composed of ministers of the word and ruling elders, with all ministers of the word and ruling elders being of equal authority for rule in the church, and with assemblies at the local level that are in subordination to higher assemblies in which the local assemblies are represented.
One of the mandated functions of this government is to perpetuate the form of sound words, committing to faithful men the task of teaching the same doctrine to others after them.
To accomplish this, it is appropriate for the church to adopt a digest of biblical doctrine, for testing the soundness of candidates for church office. Only those men are admitted to office who can declare by solemn vows that they believe the whole doctrine of the church's confessional statement to be the truths of God, that they acknowledge it as a confession of their faith, and that they will assert, maintain and defend that doctrine. Inasmuch as the vows are administered for the purpose of ensuring that the church's principles will be maintained, it is not morally conscionable that rulers who were admitted to office on such terms should afterwards overturn the church's commitment to the principles referenced in their vows.
A significant aspect of the classical Reformed confessions is the range of issues left undefined. This latitude makes it possible for numbers of men to take the vows of subscription required of church officers, despite a diversity of opinion on many points of biblical interpretation. This measured approach to concurrence about doctrine, leaving room for each minister to teach his own interpretation of Scripture on points not addressed by the church's confession, is to be accompanied by a concentration of the pulpit ministry on the primary truths of Scripture. The effect on the church of dwelling much in the main things of Scripture is a biblically proportioned spirituality.
A second function of the church's government is to supply shepherding care or spiritual oversight for the Lord's people, watching for their souls as those who must give an account. The discharge of this responsibility warrants the preparation of agreed rules and forms of procedure, which should go no further than reiterating and implementing principles laid down in the Word of God for a just and equitable administration in the church.
In acting on such premises, a church establishes a constitution. A select body of principles is assigned an elevated status, securing the church's posture on a number of points of doctrine and practice. The constitution, some of whose elements are often referred to as subordinate standards, is situated in a middle position between, on the one hand, the supreme authority of the Word of God, the truths of which the constitution professes to represent and guard, and, on the other hand, the inferior status of the church's ongoing decisions, which, if they are to be competent, must be in accord with the constitution. The historic identity of a church is found not in the preferences of transient majorities, but in its constitution, where the church has made the fundamental confession of its faith.
John Murray: The Westminster Standards
Sherman Isbell, from The Master's Trumpet, issue 3: The Church in Relation to Its Constitution (pdf)