Several interconnected reasons. Basically, Christianity like any other religion is based on identification via group. It defines those within the group and those outside. In order to do that, there needs to be indentifiable characteristics for both sides. A truly sovereign God who alone "saves", without the participation of the "saved" would offer no such markers... so this cannot be accepted.
Interesting view. Do you think this is a necessary result of being Christian, or just a common one?
Can you imagine that you would think different, if you were subjected to a group who teached and explained that "traditional views of substitutionary atonement"?
Sorry, this question doesn't make sense to me, but I will take am stab at it. If you are asking whether I should have expected any different than what I have, then no, I see my understanding of the words in these materials to be totally the result of what those words have been used to mean to me, as consistent patterns in my past. If you are asking if I am able to break that understanding and apply new meanings to the same words and phrases I an misunderstanding, yes I do think that is possible, but I will need educating accordingly. I expect these missionaries have used these words because they appeal naturally to those who accept substitutionary atonement doctrines. Most Christians are of that persuasion.
That now is difficult... potentially logically impossible. It's so much easier to show the opposite.
OK.
That is a wrong deductive conclusion. You would have to show your premise to be correct first.
OK, let's come back to that once we have agreed to a definition of sin.
What I "ever had" is irrelevant. I do not now, this is enough.
I do not agree with this. The reason I do not agree with it, is because sin is not a constant problem, but it is only a problem that occurs at times. Just as desiring fruit. If for some reason you found that desiring fruit was a problem, and sometimes you desired fruit, then your desire for fruit is a problem. You might not desire fruit right now, but you have in the past, and there is no real guarantee that you will not again desire fruit.
Not that leads us into a connundrum: assuming that I am "deliberatly dishonest" (short: lying) is just a cop-out.
Not necessarily. It sure is a common tactic for some people. In fact, I believe everyone does this to some extent sometimes. Eg, speeding when they know a police is not watching, yet when they see police, they will be honest about their speed. Because people calculate that the truth can be concealed, if the truth is not convenient to them, they can conveniently choose to conceal the truth, knowing they cannot be proven to be lying. I know it happens, so it definitely is not a cop-out. It is a serious possibility to be investigated.
As for the understanding of "sin": maybe my understanding of sin is superior to yours?
You have added a judgment to this that is not necessary, and it seems to indicate this probably is what you feel could be the right idea to investigate.
It is a false example... we have already shown that. A text will only produce expected results "when properly performed" AND is started from a accepted and verified point of origin.
You have not shown that it is a false example. You have shown that you are not following instructions properly, by using faulty equipment in your test.
Assumptions are fallible. In order to be used as a valid premise, they need to be verified (or axiomatic, which is not the case here). And verification is what we expect from the test... that would be circular reasoning, wouldn't it?
Yes. But it would produce proof beyond reasonable doubt, which you might have not had before performing the test.
You need to change your basic assumptions about Her.

Could it be that you are reluctant to take this test, because you like to keep your sinful ways?
No, you need to stop using fallacies and use convincing arguments. IPU is designed to be a straw man.