Here's your problem Luke. Every motive of man is sinful. Every one, because every motive springs from a corrupt heart. So you kind of play into a noncalvinist's hand when you make the argument that motive determines whether an act is sin or not. (I know what you mean, but thy don't think the same way, or they'd be Calvinists.)
When speaking with noncalvinists, you have to keep in mind that they don't see men as we see them. They see them as basically good though defective. They believe that a man can bring good things out of the treasures of a corrupt heart.
It's best to stick to the texts of the Scriptures they think they believe, that by the law is the knowledge of sin, not by the motive. Rahab's act did not violate the law. She did not bear false witness against her neighbor, She did what she did out of the fear of the LORD, and had no other God beside Him. It was an act of faith.
Yea, I agree with everything you said here. What you are probably not aware of is how this motive thing got started. Once you see that I think you'll see the sense in my line of argument.
I know the hearts of men are only evil continually.
But this is about God and WHY God cannot sin.
The ultimate answer is this: sin is doing something with an improper motive. God ultimately only ever has the highest of motives, his own Glory, for all that he does. Therefore he cannot sin.
This is not about the motives of men which you and I agree are ALWAYS evil.
This is about what makes a sin a sin.
This is the very reason that EVERYTHING that unregenerate men do is a sin. Even the PLOWING of the wicked is sin because sin is all about motive.