An excerpt from Richard A. Muller, Post-Reformation Reformed Dogmatics: The Rise and Development of Reformed Orthodoxy; Volume 4: The Triunity of God (Grand Rapids, MI: Baker Academic, 2003), 143–144.
"Whether positively in their creedal and catechetical expositions and their various manuals or bodies of doctrine or in their responses to the early antitrinitarians, the Reformers uniformly gave testimony to the importance of the doctrine of the Trinity in their theology and to the necessity of respecting the mystery of the doctrine. The doctrine of the Trinity, according to the Reformers and their successors, belonged to the category of fundamental and necessary articles. Even Melanchthon’s early exclusion of the doctrine from his set of treated loci (albeit not from his listing of foundational topics) had more to do with issues related to speculation than with the question of necessary beliefs for Christians. When Melanchthon added an exposition of the doctrine to later editions of his Loci communes, the trinitarian model became the central issue of his doctrine of God, given far greater detail than the unity of essence and the attributes, for, as Melanchthon testifies, although the mystery of the Trinity is beyond all human comprehension, some attempt at expressing the doctrine must be made in order to distinguish Christian worship from that of the pagans.
In the confessions and theological systems of the Reformers the doctrine of the Trinity is almost invariably placed in the creedal or catechetical order, early on in the order of doctrines, preceded only by preliminary matters and the discussion of Scripture, and followed immediately by creation and providence. Insofar as many of these works do not have extended discussions of the divine essence and attributes, the doctrine of the Trinity receives the central place in the doctrine of God."
