My 2 cents:
Don't bother trying to argue scripture and doctrine, you're damnd if you do and damned if you don't. Their circular reasoning is so "pat" in their own minds they simply won't understand your point, or will refuse to even consider it. If, by some miracle, you can get one to think and consider what you are saying, and it starts to make sense, then they will likely become even more defensiive becuase they fear they are being successfully decieved by Satan.
If I were you right now, I would ask a series of questions that will lead them to see the problem with the UN fiasco. Something like this:
Q - Is it true that JWs cannot have jobs like policeman (carrying a gun) or work in a shop at the tobacco counter (selling "Satan-sticks") or work as say, a secretary or librarian for a church of Christendom or a political party?
A - Yes
Q - Is it true that if a supposedly faithful JW went out and got one of these jobs, they are not acting in accord with scripture? Even if they did it out of necessity (e.g. to feed their family)?
A - Yes
Q - And if such a one held this type of job for 10 years and was then discovered by the congregation, there would likely me a JC formed to determine (1) if it is true and (2) are they repentant? -- just like any other reported case of serious wrongdoing?
A - Yes
Q - And if upon discovery, that bro/sis quit the job, but at the same time tried to minimize their wrongdoing and did not show signs of repentence, then that would indicate a seriously flawed, un-christian conscience?
A - Yes, that sounds like they are not repentent of their actions
Q - And those that are not repentent of serious wrongdoing are to be either "marked" or perhaps even disfellowshipped, to protect the purity of the congregation?
A - Yes, we are to protect the spirituality of the congregation.
Q - Even if that one is prominent in the cong, or a close family member, then we must mark or perhaps even DF?
A - Yes.
Q - That is why I must turn away from the Witchpower Babble & Trick Society, to protect my spirituality. They made an alliance with the UN, and are unrepentant about it.
{Then show them copies of letter from the SOCIETY "explaining" their involvement. Don't bother with anything not from the Society, it will be ignored as "Apostate lies". But letters on Society letterhead are much harder to ignore}.
You can get scans of these letters at many web resources including http://quotes.jehovahswitnesses.com/united_nations.htm
One more thing. If possible, spend $100 on a microcassette recorder with remote active mic, and wire yourself for sound recording before any conversation. Trust me on this, you will be glad you did!
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For interesting Watchtower Society literature quotes, complete with references but without any editorial, check out:
http://Quotes.JehovahsWitnesses.com
