Luther says in a Table Talk:
"consideration must be given to the revealed God on the one hand and to knowledge of the unrevealed [hidden] will of God on the other. Of the latter nothing at all is known to us. We must confess that what is beyond our comprehension is nothing for us to bother about. [...God] says in effect: "Let me remain hidden. Otherwise you will fall into the abyss of hell, as it is written, 'He who inquires into the majesty of God shall be crushed by it.' [Prov26:27] In this place leave me untouched. Carnal wisdom shall here have its limit. Here I wish to remain unrevealed. I shall reveal your election in another way. From the unrevealed God I shall become the revealed God. I shall incarnate my Son and shall give you one who will enable you to see whether you are elected. do this: Give up your speculations which are apart from the Word of God, thoroughly root them out, and drive them to the devil in hell. ' This is my beloved Son. Hear ye him'.["...] Christ says, "He that hath seen me hath seen the Father." [John 14:9] Christ will lead you to the hidden God."
(pp.132-133, Luther: Letters of Spiritual Counsel, Translated and edited by Theodore G. Tappert).
Further reading on the hidden and revealed will of God can be found in Calvin's Institutes in book 1 chapters 17 and 18 (especially in the first few sections of each chapter)
Helpfully this is when Calvin is dealing with God's providence which includes him being in some way behind evil acts, whilst his will is clearly for good acts. Predestination to salvation really follows from this less contested teaching.
