"You're at a party. How would you argue for the existence of God?"

I don’t go to parties because no one wants to hear about ontology and first causes. If they did, I might say the following.

Limited, composite and changing things have being, but are not essential or self-actual being.

(This is self-evident; a horse or tree is not being as such, it is a participated mode of being. By way of reductio, essential, self-actual being has no extrinsic differences; there is nothing outside being. So, if x were essentially being, y would be nothing, nor would there be anything for x to change into or become. So, if a thing becomes or has extrinsic differences, it is not essentially or self-actually existent.)

If the being of limited, composite and changing things is not actual according to the essence of these things, then it is extrinsically actuated.

[And it is also clear that only self-actual existence, God, can actuate what all else is merely in potency to; not even an eternal infinitude of things essentially in potency to being would amount to actual being].