The Westminster Assembly of Divines was a council of divines (theologians) and members of the English Parliament appointed from 1643 to 1653 to restructure the Church of England. Several Scots also attended, and the Assembly's work was adopted by the Church of Scotland. As many as 121 ministers were called to the Assembly, with nineteen others added later to replace those who did not attend or could no longer attend. It produced a new Form of Church Government, a Confession of Faith or statement of belief, two catechisms or manuals for religious instruction (Shorter and Larger), and a liturgical manual, the Directory for Public Worship, for the Churches of England and Scotland. The Confession and catechisms were adopted as doctrinal standards in the Church of Scotland and other Presbyterian churches, where they remain normative. Amended versions of the Confession were also adopted in Congregational and Baptist churches in England and New England in the seventeenth and eighteenth centuries. The Confession became influential throughout the English-speaking world, but especially in American Protestant theology.

The Assembly was called by the Long Parliament before and during the beginning of the First English Civil War. The Long Parliament was influenced by Puritanism, a religious movement which sought further reform of the church. They were opposed to the religious policies of King Charles I and William Laud, Archbishop of Canterbury. As part of a military alliance with Scotland, Parliament agreed that the outcome of the Assembly would bring the English Church into closer conformity with the Church of Scotland. The Scottish Church was governed by a system of elected assemblies of elders called presbyterianism, rather than rule by bishops, called episcopalianism, which was used in the English church. Scottish commissioners attended and advised the Assembly as part of the agreement. Disagreements over church government caused open division in the Assembly, despite attempts to maintain unity. The party of divines who favoured presbyterianism was in the majority, but the congregationalist party, which held greater influence in the military, favoured autonomy for individual congregations rather than the subjection of congregations to regional and national assemblies entailed in presbyterianism. Parliament eventually adopted a presbyterian form of government but lacked the power to implement it. During the Restoration of the monarchy in 1660, all of the documents of the Assembly were repudiated and episcopal church government was reinstated in England.

The Assembly worked in the Reformed Protestant theological tradition, also known as Calvinism. It took the Bible as the authoritative word of God, from which all theological reflection must be based. The divines were committed to the Reformed doctrine of predestination—that God chooses certain men to be saved and enjoy eternal life rather than eternal punishment. There was some disagreement at the Assembly over the doctrine of particular redemption—that Christ died only for those chosen for salvation. The Assembly also held to Reformed covenant theology, a framework for interpreting the Bible. The Assembly's Confession is the first of the Reformed confessions to teach a doctrine called the covenant of works, which teaches that before the fall of man, God promised eternal life to Adam on condition that he perfectly obeyed God.

The Parliament called the Westminster Assembly during a time of increasing hostility between Charles I, monarch of England and Scotland, and the Puritans. Puritans could be distinguished by their insistence that worship practices be supported implicitly or explicitly by the Bible, while their opponents gave greater authority to traditional customs. They believed the Church of England, which had separated itself from the Roman communion during the English Reformation, was still too heavily influenced by medieval Catholicism. They sought to rid the church and nation of any of these remaining influences. This included the Church's episcopal polity, or rule by a hierarchy of bishops. Non-separating Puritans, unlike Separatist Puritans, did not leave the established church. Under Charles, the Puritans' opponents were placed in high positions of authority, most notably William Laud who was made Archbishop of Canterbury in 1633, even though these "high churchmen" were in the minority. Puritans were forced to keep their views private or face fines and imprisonment. Laud promoted advocates of Arminianism, a theological perspective opposed to the Reformed theology of the Puritans. Worship practices such as kneeling at communion, bowing at the name of Christ, and the placement of communion tables at the East end of churches were also reinstated. To the Puritans, these seemed to be a step in the direction of Catholicism.

There were also conflicts between the king and the Scots, whose church was ruled by a system known as presbyterianism, which features elected assemblies. James, Charles's predecessor as King of Scotland, made it clear that he intended to impose elements of episcopal church government and the Book of Common Prayer on the Scots beginning in 1604. The Scots considered this a reversion to Roman Catholicism. Charles furthered English impositions on the Church of Scotland in 1636 and 1637. This led to the First Bishops' War between Charles and the Scots in 1639. Charles called what came to be known as the Short Parliament to raise funds for the war, but he soon dissolved it when it began voicing opposition to his policies. Following the Second Bishops' War with the Scots in 1640, Charles was forced to call another parliament to raise additional funds.

What came to be known as the Long Parliament also began to voice vague grievances against Charles, many of which were religious in nature. Parliament had many Puritans and Puritan-sympathizing members, who generally opposed the existing episcopal system, but there was little agreement over what shape the church should take. Later in 1640, the Root and Branch petition was presented to the House of Commons, the lower house of Parliament. It was signed by about 15,000 Londoners and called for total elimination of the episcopal system. Committees were organized in the House of Commons to enact religious reforms, leading to the imprisonment of Archbishop Laud and his supporters in the Tower of London as retaliation for their repression of Puritans. The Court of High Commission and the Star Chamber, courts which had inflicted severe punishments on Nonconformist Puritans, were also abolished.

The idea of a national assembly of theologians to advise Parliament on further church reforms was first presented to the House of Commons in 1641. Such a proposal was also included in the Grand Remonstrance, a list of grievances which Parliament presented to Charles on 1 December that year. Charles responded on 23 December that the church required no reforms. Undeterred, Parliament passed three bills in 1642 appointing an assembly and stipulating that its members would be chosen by Parliament. Charles, whose royal assent was required for the bills to become law, was only willing to consider such an assembly if the members were chosen by the clergy. This was the practice for selection of members of Convocation, the assembly of clergy of the Church of England.

Defying the king, between 12 February and 20 April 1642, each county delegation of England in the Commons chose two divines, in addition to two for each county of Wales, four for London, and two for each University (Oxford and Cambridge). County delegations often chose divines from their own county, but not always. Commons chose the members in this way to ensure that their local constituencies were represented in the decision. The House of Lords, Parliament's upper house, added another fourteen names on 14 May, to which the Commons agreed. Meanwhile, relations continued to deteriorate between the king and Parliament. Charles raised the royal standard at Nottingham on 22 August, marking the beginning of the First English Civil War. The start of the war lent support to the cause of the Assembly in Parliament, because holding it would convince the Scots that Parliament was serious about reforming the church and induce the Scots to come to their aid. On the other hand, the war added strain to the already busy Parliament.