The Westminster Assembly was called by ordinance of both Houses of Parliament and met for the first time on July 1, 1643. Nearly all the sessions were held in the Jerusalem Chamber in Westminster Abbey.

The first work which the Assembly undertook was the revision of the Thirty-Nine Articles of the Church of England. On October 12, 1643, when the Assembly was engaged in the revision of the sixteenth Article, there came an order from both Houses of Parliament to treat of such discipline and government as would be most agreeable to God's Word, and most apt to procure and preserve the peace of the Church at home and nearer agreement with the Church of Scotland and other Reformed Churches abroad. and also to treat of a directory for worship. It was in pursuance of this order that the Assembly prepared what are known as 'The Form of Presbyterial Church Government' and 'The Directory for the Public Worship of God'.