How interesting Milton's use of certain is. In particular the difference between God in Book 3 and Adam in Book 9.

God:

if I foreknew,
Foreknowledge had no influence on their fault,
Which had no less prov'd certain unforeknown.

Yes, maybe. This is close to the God-outside-of-time idea that Hobbes had such contempt for, since you can only explain it consistently with free will if God's attitude towards the future is the same as ours towards the past: the choice was or will be free but it was or will be made.

And then there's Adam:

som cursed fraud
Of Enemie hath beguil'd thee, yet unknown,
And mee with thee hath ruind, for with thee
Certain my resolution is to Die....

A different kind of certainty this: a moral certainty, a commitment in the mode of certainty. If the echo is intentional, and I think it is, it reflects little credit on God.

Since I've been thinking about On Certainty and "Other Minds," I'll add that I think there's a different kind of feel to "certain" as predicated of events from the sense you get when it's predicated of those asserting belief or knowledge. Wittgenstein tends to merge certainty and knowledge a little, Austin to distinguish between them. If I claim to know something, you can ask me how I know it; if I claim to be certain of something, you can ask me why I am certain. To this I would add a third idea and a third interrogative -- not an adverb but a pronoun this time. If something is certain (if it "denotes a foregone conclusion") we can ask, what is it that is certain? That kind of question is often one about how much you know when you know something is going to happen ("Death is certain," or "inevitable," to quote Nabokov's grammar-book example) -- you may not know what that something is, but its opacity is a blackbird-part of what you know.

For God, the fall/fault is certain and nothing else matters; for Adam the certainty is that he will pitch himself into the uncertain world of the fall: all he is certain of is that that's what he's going to do. I think he is seeing that he embraces his immediate intentions so strongly that they don't have to tell him anything, don't offer themselves as a subject of deliberation. If he's offering certainty, if this is a performative claim, it's that he's telling her (the internalized her since he's speaking inwardly), that she can be certain that he'll do this.

I think that's why when he finally talks to Eve he puts it in a subtly different way--

However I with thee have fixed my lot,
Certain to undergo like doom--

which now does describe his conviction of the result (whether "bliss or woe") of his doing it. He's doing it even in the face of his certainty that he'll be punished as she is, so it's a certainty about the result of his resolution, or about the effect of his resolve, and not just the resolve itself.

(Even here though, he's not saying he's certain he'll die: he's saying he's certain that the result of doing what Eve is doing is that he will share her fate -- "if death / Consort with thee death is to me as life.")