Luddite, in the most general sense, could be seen as a kind of religion (well, not really, but kind of), and you can most certainly consider yourself a minority.
So, the likely best way to deal with this would be to politely let the screener know that you do not want to dicuss or justify your beliefs, and you ask him not to interfere further with your human right of self-fulfilment and freedom of faith. Few people are too dim to not take the hint, and few want to get into a discussion which is going that way.
Shutting down the discussion fast and early is important because you cannot win. Not only can you not win because (other than it may appear) there is no discussion happening there. A discussion is about exchanging points of view, and the screener isn't interested in your point of view at all. He wants you to do the body scan, nothing else. Also, you are a rookie who is having a discussion with a professional. Mind you, that guy is having that same discussion ten times every day, and dim-witted as he may be, he's heard all the arguments before, you couldn't win a discussion if there was one.
That being said, I am not going to ask in what way boarding an airplane is compatible with being opposed to modern technology.
What I have been doing for years (though, amusing as it can be, I grew tired of dealing with these people, so now I'm simply not flying any more) is to make such a scene that they're finally happy if only you move on already. Note that this is possibly not the smartest thing to do unless you bring an extra hour of time. If you miss your flight, that's bad luck for you (hasn't happened to me, but could very well).
Staying firm but polite is almost certainly the better choice.
On the one hand side, there is no legal base for the screeners to have you undergo a body scan, much like there is no legal base for any items to be "confiscated" (i.e. stolen) by them.
However, on the other hand side, you want to get to that gate. And, guess what, you have no legal base to get through to your gate unless the examiner is satisfied, whatever that means. Yes, they can't force you to take the body scan, but they can refuse you entry. Yes, the scan (and the entire examination) is complete bull. No, it doesn't have anything to do with security.
But whatever it may be, finally the only thing that counts for you is: Do you get to your plane before it leaves, or do you not. Airplane gone, bad luck for you, no money back.
So, the smartest thing to do is probably to stay calm and polite, but firmly explain that the body scan is not a viable option for you, and you do not wish to justify or discuss your beliefs. Or, well, don't fly, which is what I'm doing now.