‘Because it’s mine’ is not an uncommon view, Harry. Nobody refers to me as the arbitrer of property rights when someone wants to take something that isn’t theirs.
But you’ve already discounted the concept of ownership as a reason why someone shouldn’t steal (although it’s one of the bedrock principles). That’s why we eagerly await your suggestion for another reason why a moral position should be held.
In the meantime, as an small interlude, we can imagine a short scenario:
F: That’s a nice pen. I’m having that.
H: Where’s my pen?
F: Here it is. I’m taking it with me.
H: You can’t do that!
F: Why not?
How does that pan out, Harry? Do you give him a theological argument or do you say ‘You can’t take it. Because it’s MY pen!’
Guess it’s not the latter. You don’t think it’s a good argument. You don’t think it’s an argument at all.