This is a good and fair question, Caroljeen, and I will attempt to respond in-kind; I hope this doesn't get too wordy:
My first answer is that I can't. I know it as an unassailable fact, for myself - in the same way that you know with absolute surety whether you like broccoli or not. There is no doubt in your mind as to what you've experienced/concluded, even though you can't prove this experience/conclusion to the people around you.
My second answer, however, is that I have no burden to disprove the existence of something for which there's no proof of in the first place. Oseas - and in practice most Christians - feel no need to prove to others that their God exists. Indeed, the standard of proof that they use amongst themselves appears to be nothing more than faith: they trust that their belief is correct. In this context. I should be able to state my beliefs as unassailable facts, too, and ignore those who demand an evidentiary standard from others that they reject for themselves.
Finally, my unassailable knowledge is based on simple logic:
- A God who is Love would not be the petty vengeful deity Oseas seems to worship
- A God who creates entire universes wouldn't condemn to eternal torture those who fail to believe in Him, or tell lies, or who use their genitals in ways not on the "Approved" list. Such behaviors might indeed be bad/wrong, but the kind of punishment such things require would never be the kind Oseas believes in
- A God who wants that none should perish would never remain silent and invisible. He wouldn't present as evidence for His existence ambiguous things which could point to any one of a hundred other gods as well. He would make himself known objectively, and take the responsibility of spreading that knowledge away from sinful / flawed (and often hateful) believers
There are many other reasons I know Oseas' god is a fiction, but the above is at least a decent starting point for any kind of proof I might share.
I'd like to point out one last thing, though:
I don't believe you and Oseas worship the same deity. I'm pretty sure you both consider yourselves to be bible-believing Christians, but despite this, you both describe the same deity quite differently. So different, in-fact, that you cannot both be right about His nature. At least one of you is wrong about Him.
As such, I don't necessarily know the God you worship is a fiction. I do know that Oseas' is, though.