It has become a standard argument from those who believe in the absolute sovereignty of God to speak of two wills of God – the revealed, normative word, and the inscrutable, positive decree. I first encountered the idea from John Piper. Jonathan Edwards and Saint Augustine (City of God, 22.2) have also written on the subject. All of these are convincing enough in their own right, but I like the logical distinction best which is provided by Martin Luther, commenting on Ezekiel 18:23 in The Bondage of the Will:
We must discuss God, or the will of God, preached, revealed, offered to us, and worshipped by us, in one way, and God not preached, nor revealed, nor offered to us, nor worshipped by us, in another way. . . .
Now, God in his own nature and majesty is to be left alone; in this regard, we have nothing to do with him, nor does he wish us to deal with him. We have to deal with him as clothed and displayed in his word, by which he presents himself to us. That is his glory and beauty, in which the Psalmist proclaims him to be clothed (Ps. 21:5). . . . God preached works to the end that sin and death may be taken away, and that we may be saved. ‘He sent his word and healed them’ (Ps. 107:20). But God hidden in majesty neither deplores nor takes away death, but works life and death, and all in all; nor has he set bounds to himself by his word, but has kept himself free over all things.
. . . The Diatribe [against which this book was written] . . . makes no distinction between God preached and God hidden, that is, between the Word of God and God himself. God does many things which he does not show us in his word, and he wills many things which he does not in his word show us that he wills. Thus, he does not will the death of a sinner – that is, in his word; but he wills it by his inscrutable will. At present, however, we must keep in view his word and leave alone his inscrutable will; for it is by his word, and not by his inscrutable will, that we must be guided. In any case, who can direct himself according to a will that is inscrutable and incomprehensible? . . .
So it is right to say, “If God does not desire our death, it must be laid to the charge of our own will if we perish”; this, I repeat, is right if you spoke of God preached. For he desires that all men should be saved, in that he comes to all by the word of salvation, and the fault is in the will which does not receive him, as he says in Matt. 23:37. . . . But why the majesty does not remove or change this fault of will in every man (for it is not in the power of man to do it), or why he lays this fault to the charge of the will, when man cannot avoid it . . . as Paul says in Rom. 11: “Who art thou that repliest against God?”
The necessary distinction is between God behind reality and God in front of reality, stepping into that which he has himself entirely wrought as if he were a part of it. It is between the word and the substance of God; or, one might say, between the Son and the Father. Read the latter half of Luther’s second paragraph and then say there is no appreciation in the Reformed tradition for mystery! One cannot but be awed by the inexorable working of God in history to his own glory.