The discussion of “archetypal” and “ectypal” theology seems to follow from an understanding of Deuteronomy 29:29:
“The secret things belong to the Lord our God, but the things that are revealed belong to us and to our children forever, that we may do all the words of this law…”
Muller moves from a discussion of the Medieval definitions to Reformation-era usage. He believes this is sparse, but also (as indicated earlier), that such a distinction exists in the Reformers. He fleshes that out a bit here.
At first glance, the Reformed orthodox doctrine of archetypal and ectypal theology has little background in the era of the Reformation. The language of the distinction has, most obviously, medieval resonances. Still, although the language itself probably cannot be found among the Reformers, there are substantive parallels to the meaning and intentions of the distinction.
Beginning with Luther, the Reformation had a strong sense of the transcendence of God, indeed, the hiddenness of God in and behind his revelation.
Drawing on this assumption, Calvin argued the accommodated nature of God’s revelation: God reveals himself not as he is in his infinite majesty but in a form accessible to human beings. So too is a distinction made by Calvin and others between the eternal decree of God and its execution in time—accompanied by the proviso that human beings can never enter the ultimate mind and will of God to discern its contents but must trust in what has been revealed and must gain assurance from the revelation of Christ and from his work in the hearts and minds of God’s people.*
Thus, the theology of the Reformation recognized not only that God is distinct from his revelation and that the one who reveals cannot be fully comprehended in the revelation, but also that the revelation, given in a finite and understandable form, must truly rest on the eternal truth of God: this is the fundamental message and intention of the distinction between archetypal and ectypal theology.
Muller, R. A. (2003). Post-Reformation Reformed Dogmatics: The Rise And Development Of Reformed Orthodoxy; Volume 1: Prolegomena To Theology (2nd ed., p. 229). Grand Rapids, MI: Baker Academic.
* Elsewhere, Muller notes:
Calvin never tires of arguing that man is incapable of reaching, grasping, comprehending the divine. He disallows speculation concerning the nature of God or of God’s decree. Christians must look at Christ and know God in him rather than “wander through … speculation and seek above the clouds”. We do not comprehend the divinity of the son but only its revelation in the flesh. We do not see the full glory of God, but rather acknowledge its presence, hidden and lowly, in the incarnation by the grace of the Holy Spirit….
In a famous passage of the Institutes Calvin notes that man’s curiosity seeks “forbidden bypaths” when it attempts to penetrate to the secret will of God:
For it is not right for men unrestrainedly to search out things that the Lord has willed to be hid in himself, and to unfold from eternity itself the sublimest wisdom, which he would have us revere but not understand, that through this also he should fill us with wonder. He has set forth by his Word the secrets of his will that he decided to reveal to us. These he decided to reveal insofar as he foresaw that they would concern us and benefit us (Institutes 3.11.1).
Cited in Muller, “Christ and the Decree”, 2008 Revised Edition, Grand Rapids, MI: Baker Academic, © 1986, pgs 20-21.