"What do Reformed churches believe? It is tempting to answer this question by trying to figure out what well-known and contemporary Reformed teachers, pastors, and professors believe the Bible to teach, but Reformed churches have a more accessible and objective way to answer this question, namely, the church's creeds and confessions. The sole foundation of every creed and confession adopted by Reformed churches is the Bible. Because we believe that the Bible is the inspired, inerrant Word of God, the only rule of faith and obedience, we therefore confess only what we find expressly taught in the text of Scripture or what we can necessarily infer from the teaching of Scripture. Some creeds and confessions are unique to Reformed churches. These include the Three Forms of Unity (the Belgic Confession, the Heidelberg Catechism, and the Canons of Dort) and the Westminster Standards (the Westminster Confession of Faith and the Westminster Larger and Shorter catechisms)."
Guy Prentiss Waters, Foreword to Welcome to a Reformed Church: A Guide for Pilgrims by Daniel Hyde (p. xiv). Kindle Edition.