Principia in the Confessions
...the confessions present the cognitive foundation or principium cognoscendi of revealed theology, the Holy Scriptures, and, based upon Scripture, the essential foundation or principium essendi of all theology, which is to say, God himself. Without the former, theology could not know the truth of God—without the latter, there could be no theology, indeed, no revelation. The movement of faith from one principium to the other is noted explicitly by the Belgic Confession
Richard A. Muller, Post-Reformation Reformed Dogmatics: The Rise and Development of Reformed Orthodoxy; Volume 2: The Cognitive Foundation of Theology, 2nd ed. (Grand Rapids, MI: Baker Academic, 2003), 151-152.