Indeed, vanity joined with pride can be detected in the fact that, in seeking God, miserable men do not rise above themselves as they should, but measure him by the yardstick of their own carnal stupidity, and neglect sound investigation; thus out of curiosity they fly off into empty speculations. They do not therefore apprehend God as he offers himself, but imagine him as they have fashioned him in their own presumption. When this gulf opens, in whatever direction they move their feet, they cannot but plunge headlong into ruin. (p. 48-49, Institutes of the Christian Religion)"By the yardstick of their own carnal stupidity". Today, this is rampant - we want God to act in a way that is consistent with modern sensibilities, or perhaps as one who is limited in his nature in a way like us. We make God out to be someone who shares our frailties, for whatever reason. Others, for example, who are unwilling to allow God to transcend our ability to neatly explain His character, and are unwilling to simply let Scripture speak for itself (e.g. those who refuse to believe in the doctrine of the Trinity) thereby create a god of their own construction.
Indeed, whatever they afterward attempt by way of worship or service of God, they cannot bring as tribute to him, for they are worshiping not God but a figment and a dream of their own heart. Paul eloquently notes this wickedness: "Striving to be wise, they make fools of themselves" [Rom. 1:22 p.]. He had said before that "they became futile in their thinking" [Rom. 1:21]. (p. 48, Institutes of the Christian Religion)This is exceedingly dangerous - and must be with pleading and concern for the soul withstood in our friends. Calvin warns, in section 3 of chapter 4, that fashioning God according to our own whim is strictly forbidden.
For they think that any zeal for religion, however preposterous, is sufficient. But they do not realize that true religion ought to be conformed to God's will as to a universal rule; that God ever remains like himself, and is not a specter or phantasm to be transformed according to anyone's whim. (p. 49, Institutes of the Christian Religion)Zeal ill placed is not worthy of praise, but worthy of deep concern for the souls of those so zealous. When we fail to let Scripture speak for itself, and tell us exactly who God is in exhaustive detail, in such a way as we are able to comprehend Him fully, we have molded our own god for ourselves. Mysteries like the Trinity, and the hypostatic union are essential doctrines - they reflect what God has revealed of Himself in Holy Scripture - and they will not be completely understandable. The fact of one God, three persons, and that of one person, two natures, fail to fit into our measly brains - and we must be content with this fact, and not demand to be able to mathematically fit God into a functional form that is completely comprehensible. Such a demand is evidence, I think, of a lack of submissive attitude and an unwillingness to fully bend the knee to God.
It is easy to point out errors like this, and label them as such - but let us always strive, for our own understanding of God, to subject ourselves FULLY to the rule of Scripture. May we repent of our erroneous human constructions and let God speak for Himself. He has revealed Himself perfectly in the infallible Word - and where there are things too lofty for us to fully understand, may we always be willing to bow before Him, and leave it at that.