Parliament summoned the Westminster Assembly in 1643, in the middle of the English Civil War, to advise on reforming the doctrine, worship and government of the Church of England along more thoroughly Reformed lines. It sat for nearly six years at Westminster Abbey, drawing together over a hundred English clergy along with a smaller group of Scottish commissioners whose support Parliament needed against the king and who pressed hard for Presbyterian church government rather than the compromise many English members preferred.
The Confession itself was largely finished by 1646, after the Assembly had already produced a Directory for Public Worship and forms of church government, and its thirty-three chapters move from Scripture and the decrees of God through creation, providence, the fall, covenant and Christ's mediation to justification, the church, the sacraments, church censures, synods and the last judgment, a complete Reformed system rather than a response to one dispute.
Two shorter catechisms, the Larger and the Shorter, followed to put the same doctrine into a form for teaching. The Assembly worked entirely under Parliament's authority and never had power to impose anything on its own; whether its work would govern the English church at all still depended on a political settlement that the war had not yet decided.