Human is a very abstract quality, isn't it? It's a bit ironic that the monsters considered antithetical to humanity are sustained by human fear, I've always thought. The line is explicitly blurred in canon, which we see as early as with Jane Prentiss and her terrified devotion.

But I think I know what you mean. Pain, death, and fear are immanent to the human condition; the desire to transcend these shackles is, therefore, the drive to become inhuman (this must not be equated with being evil; the only reason why Jonah's actions can be considered immoral is that his method for approaching divinity requires the suffering of others, not that there is something intrinsically wrong with not wanting to be human or an inherent moral purity or impurity in humanity). Ironically, this urge, actuated by fear, is in itself very human.

So, I'd say that the closer Jonah becomes to freeing himself of the weight of death the further away he gets from his humanity, and vice versa. Jonah is, in my opinion, a human person trying to carve himself into a god, and the closest he comes to his goal is when the Beholding makes him a part of its apotheosis. He becomes one with the sickening dread that has always haunted him, but it doesn't sink its claws into him; it merely passes through him because it recognises him as one of its own. To escape death, he must embrace it, wield it, and become it.

So I'd say Jonah is mostly human, especially since his motivation is fundamentally human. And he's at his most human in the moments after his agonised bliss is interrupted and he's faced with the reality of his imminent death; whatever criticisms I have of how that scene was written, there's something very poetic about that.