(Calvin is speaking here about the creation and nature of Angels, but the quote applies to all speculations concerning the hidden things of God not revealed in scripture.)
Let us remember here, as in all religious doctrine, that we ought to hold to one rule of modesty and sobriety: not to speak, or guess, or even to seek to know, concerning obscure matters anything except what has been imparted to us by God’s Word. Furthermore, in the reading of Scripture we ought ceaselessly to endeavor to seek out and meditate upon those things which make for edification. Let us not indulge in curiosity or in the investigation of unprofitable things. And because the Lord willed to instruct us, not in fruitless questions, but in sound godliness, in the fear of his name, in true trust, and in the duties of holiness, let us be satisfied with this knowledge.
For this reason, if we would be duly wise, we must leave those empty speculations which idle men have taught apart from God’s Word concerning the nature, orders, and number of angels. I know that many persons more greedily seize upon and take more delight in them than in such things as have been put to daily use. But, if we are not ashamed of being Christ’s disciples, let us not be ashamed to follow that method which he has prescribed. Thus it will come to pass that, content with his teaching, we shall not only abandon but also abhor those utterly empty speculations from which he calls us back.
* as an aside in the Institutes Calvin describes when someone asks a wise man, “What was God doing before He created all things?” (This story comes from Augustine.) Calvin quotes Augustine’s answer to that question by saying, “He was making hell for the curious.”