There exists a god.
A god is any being which is strictly superior to and has comprehensive authority over all of reality. (definition)
Humans exist and are autonomous. (premise)
A human possesses the power of self-determination of his thoughts and actions, exercised in exclusion to the god’s power of shaping reality. [from (3)]
Human self-determination precludes the god’s strict superiority and comprehensive authority over humans. [from (4)]
Therefore, it is false that a god exists. (by contradiction)
I don’t subscribe to this proof because I disagree that humans are autonomous. But this proof suggests why the unbeliever is committed to opposing God — because the loss of autonomy in the face of a sovereign God is odious. The existence of God is not unreasonable or unconscionable; rather, it places very real demands upon us that run counter to our sense of self-importance and our desire for self-interpretation and self-determination.
The materialist is committed to opposing God because he is committed to the idea that he is an autonomous observer and interpreter of reality. If it were possible that there were a sovereign God (or even that we might be brains in vats) then the notion of man’s objectively interpreting reality would be undone. Science would become a merely subjective and pragmatic study of how the world happens to appear to us at this moment, rather than an objective study of what truly exists.
Moreover, most unbelievers are committed to opposing God because they are committed to the idea that they are autonomous agents. But while God has granted us some degree of freedom, he has not released us from responsibility nor given us real autonomy. We will all at length be held accountable to him, on his terms rather than ours. It cannot be otherwise, for God is God: “no creature is hidden from his sight, but all are naked and exposed to the eyes of him to whom we must give account” (Hebrews 4:13). This is offensive to anyone that fancies himself the unconquered “master of my fate [and] captain of my soul” [1]. The unbeliever opposes God, not as a matter of reason from neutral principles, but as an attempt to cling to the illusion of autonomy.
How then shall we live? We are responsible and accountable to God. Far better to live and die on his terms than to attempt to do so on ours. Far better to enjoy God and the gifts he has filled this earth with, than to enjoy the gifts alone with a fearful expectation of judgment. Yet all of us rebel and chafe under God’s authority; we all need a savior, both to remove our guilt and to empower us to submit to God’s authority with joy. Praise God for providing a perfect savior in Jesus Christ!
And how shall we do science, or interpret anything for that matter? God has revealed himself to be a sovereign and orderly creator. So, far from wondering whether the meaningfulness of our observations is frustrated by whatever lies outside of our ability to see, we have a real and genuine confidence in the reality and orderliness of what we see. More than that, we have a real mandate and even responsibility to discover, responsibly use, and enjoy what God has made and done. All the world points to God, for all the world is fashioned and sustained by him.
See also Alving Plantinga on treating God as an explanation.