Mingled vanity and pride appear in this, that when miserable men do seek after God, instead of ascending higher than themselves as they ought to do, they measure him by their own carnal stupidity, and, neglecting solid inquiry, fly off to indulge their curiosity in vain speculation. Hence, they do not conceive of him in the character in which he is manifested, but imagine him to be whatever their own rashness has devised. This abyss standing open, they cannot move one footstep without rushing headlong to destruction. With such an idea of God, nothing which they may attempt to offer in the way of worship or obedience can have any value in his sight, because it is not him they worship, but, instead of him, the dream and figment of their own heart.
There really is nothing new under the sun. If I didn't know better, I would think that Calvin was writing a commentary on our culture today. Too often I hear people rejecting sound biblical doctrines because they can't imagine "their" God acting in such a way. What they don't realize is that they have allowed the culture they live in, whether it be within or outside of the church, to set parameters for God that He must now reside in. Anything that is brought to them concerning God that exists outside those parameters is immediately dismissed as being false. Instead of scripture being the authority that sets the boundaries (or lack there of) for God, their self made boundaries for God now become the test for the validity of scripture. Somehow, the idea that a fallen creature imposing limitations on the character and actions of its own Creator seems perfectly normal.
As Calvin points out, one of the major downfalls of this thinking is that we are no longer worshiping the one true God of scripture, but instead an idol of our own making with no power to actually accomplish anything in and through us. There is only one God and He has revealed Himself to us in His word. Our choice is to accept Him as He is, or not at all. He makes no concessions or apologies for who He is because He is perfect in every way. Any attempt on our part to define Him outside of His own definition is by its very nature a lessening of who He is from the Creator and Sustainer of all things into a worthless false God.