Shorter Catechism Q. 3. What do the Scriptures principally teach? A. The Scriptures principally teach what man is to believe concerning God, and what duty God requires of man.
Commentary: The puritan Thomas Goodwin said that everything in the Bible may be reduced to two points: Credenda, things to be believed, and Agenda, things to be done. More precisely, the Shorter Catechism says that these two points are what the Scriptures principally teach. While the Bible includes interesting historical and geographical details, its main message is not about such things as the identity and location of the Hittites. It is about what we are to believe about God and what God commands us to do.
These two principal parts are sometimes called indicative and imperative, creed and conduct, doctrine and duty, belief and behavior, orthodoxy (right beliefs) and orthopraxy (right practice). They also provide a broad outline for the remainder of the Catechism. Questions 4-38 cover what we are to believe about God, creation, providence, the fall, and redemption, while Questions 39-107 focus on what we are to do by addressing the Ten Commandments, faith and repentance, the means of grace, and prayer.
Both principal parts are important and necessary and must never be separated from each other. Faith without obedience is dead faith, dead orthodoxy and antinomianism. Obedience without faith is theological liberalism, moralism, and the social gospel. Moreover, there is no true faith if there is no obedience, and there is no true obedience if there is no faith. Trust and obey, for there’s no other way, because as Paul says, what matters is faith working through love (Gal. 5:6).
Older catechisms taught the Christian faith by expounding the Apostles’ Creed and Christian practice by expounding the Ten Commandments and the Lord’s Prayer (see e.g. the Heidelberg Catechism). The Westminster Assembly departed partly from this practice by omitting the exposition of the Apostles’ Creed because they wanted to base their catechism directly on Scripture instead of an exposition of Scripture. This is akin to a preacher wanting to preach from Scripture instead of from a faithful commentary on Scripture. The adoption of this new method, as Chad Van Dixhoorn has observed, enabled the Assembly to be consistent with their position that doctrine comes from the Bible alone.