Carlo Sclippa suggested this very interesting argument against agnosticism to me:
The agnostic concludes that neither the evidence for or against God is compelling. So the one thing that they think is reasonable to conclude is that the world is ambiguous concerning God. Is that fact consistent with the existence of God? Would an all powerful, all good, and all knowing being deliberately devise a world in which the evidence for God’s existence is obfuscated to the point that a reasonable person cannot form a clear view about it? That seems implausible. Such a being would certainly have the power to make the evidence clearer one way or the other. It would know how to make the world unambiguous with regard to its own existence. And presumably, if it was all good, or loved humanity, it wouldn’t leave them dangling in the wind, searching for answers but not finding them, depriving them of thing they want the most. So the agnostic has to reconcile the fact that they think the evidence is insufficient with God. God wouldn’t leave the evidential situation inconclusive. So if the evidential situation is inconclusive, then there is no God. The agnostic believes that the evidential situation is inconclusive. Therefore the agnostic should conclude that there is no God.
Here are some very powerful analogies that J.L. Schellenberg gives for why a good God wouldn’t leave the evidential situation inconclusive. This passage is from "Divine hiddenness justifies atheism," in The Improbability of God, eds. Michael Martin and Ricki Monnier.
I. The Hiding Analogy: Imagine yourself in the following situation. You’re a child playing hide and seek with your mother in the woods at the back of your house. You’ve been crouching for some time now behind a large oak tree, quite a fine hiding place but not undiscoverable—certainly not for someone as clever as your mother. However, she does not appear. The sun is setting, and it will soon be bedtime, but still no mother. Not only isn’t she finding you, but, more disconcerting, you can’t hear her anywhere: she’s not beating the nearby bushes, making those exaggerated “looking for you” noises, and talking to you meanwhile as mothers playing this game usually do. Now imagine that you start calling for your mother. Coming out from behind the tree, you yell out her name, over and over again, “Mooooommmmmmm!” But no answer. You look everywhere: through the woods, in the house, down the road. An hour passes, and you are growing hoarse from calling. Is she anywhere around? Would she fail to answer if she were around?
Now let’s change the story a little. You’re a child with amnesia—apparently because of a blow to the head (which of course you don’t remember), your memory goes back only a few days—and you don’t even know whether you have a mother. You see other children with their mothers and think it would sure be nice to have one. So you ask everyone you meet and look everywhere you can, but without forwarding your goal in the slightest. You take up the search anew each day, looking diligently, even though the strangers who took you in assure you that your mother must be dead. But to no avail. Is this what we should expect if you really have a mother and she is around, and is aware of your search? When in the middle of the night you tentatively call out—“Mooooommmmmmmmm!”—would she not answer if she were really within earshot?
Let’s change the story one more time. You’re still a small child, and an amnesiac, but this time you’re in the middle of a vast rain forest, dripping with dangers of various kinds. You’ve been stuck there for days, trying to figure out who you are and where you came from. You don’t remember having a mother who accompanied you into this jungle, but in your moments of deepest pain and misery you call for her anyway: “Mooooommmmmmm!” Over and over again. For days and days. . . the last time when a jaguar comes at you out of nowhere. . . but with no response. What should you think in this situation? In your dying moments, what should cross your mind? Would the thought that you have a mother who cares about you and hears your cry and could come to you but chooses not to even make it onto the list?