I'd like to respond to one of Spencer Toy's objection from another angle:
This I think truly exposes the fatal flaw of the Calvinist’s embrace of Divine determinism. As William Lane Craig has stated, once a person embraces determinism of any sort a strange vertigo sets in. One very well may believe true things, but only because they’ve already been determined to believe those things just as much as their opponents have been determined to believe false things. In such a system, nothing can be rationally affirmed.
It isn't clear what Toy is attempting to argue. He says that if someone has been determined to believe true things while someone else has been determined to believe false things, then nothing can be rationally affirmed. But he doesn't spell out why that's the case. He puts the emphasis on determinism, but he fails to explain how that's germane.
For instance, suppose we changed this to: "Someone may accidentally believe true things just as much as their opponents may accidentally believe false things." How would that change Toy's conclusion, and why? If your beliefs are the result of chance rather than determinism, why isn't that a reason to be skeptical?
Perhaps this is what Toy is trying to get at: if each side is determined to believe what they do, then they can't tell which side is right and which side is wrong. And in that event, nothing can be rationally affirmed. If that's what he's gesturing at, I'd say a few things:
i) What's the warrant for rationally affirming something? What about if I can point to evidence for my belief? Or give reasonable explanations for my belief? In many situations, that's the best that can be expected. We don't have apodictic proofs for most of our beliefs, including many important beliefs. Certainly evidential apologetics, which Toy espouses, doesn't demand that.
ii) If I can't know that I'm mistaken, does it follow that nothing can be rationally affirmed? Take memories. We are hugely dependent on memory. Yet memory is fallible. In many cases, if I misremember something, I can't detect my mistake–because memory is all I have to go on. Although in some cases I may have access to independent information that enables me to corroborate or correct my memories, in many cases, the memory is all I have. Does it follow that nothing can be rationally affirmed on the basis if memory, just because there are instances in which I can't tell if I'm mistaken? Given our tremendous reliance on memory, that would entail a devastating degree of skepticism.