Westminster Confession of Faith
The Westminster Confession of Faith is a Reformed statement of doctrine completed by the Westminster Assembly in 1646. It is one of the most influential confessions in Presbyterian and Reformed churches and a standard primary source for what Calvinism teaches.
Expanded explanation
A confession is a church's summary of what it believes Scripture teaches. Reformed churches treat the Westminster Confession as a subordinate standard: it is meant to serve under Scripture, not alongside it. Quoting the Confession is useful because it shows what a broad tradition has formally affirmed, rather than one author's individual phrasing. Decalvinised quotes it so that Calvinist claims are evaluated in their strongest and most representative form.
Context
Not every Calvinist holds the Westminster Confession. Reformed Baptists commonly use the Second London Baptist Confession of 1689, which closely follows Westminster, and Continental Reformed churches use the Three Forms of Unity: the Belgic Confession, the Heidelberg Catechism, and the Canons of Dort. The Canons of Dort (1618–1619) are the historical source of the five points later summarized as TULIP. The same assembly that wrote the Confession also produced the Larger and Shorter Catechisms. Quoting the Confession shows representative Calvinist teaching; it does not mean that every Calvinist agrees with every article.