"you can't reason with religious people"
That's absurd. Some religious people are completely unreasonable, but the vast majority will listen to a rational argument, especially if it isn't specifically about religion. Have you heard of, say, Newton?
As for assuming that the opponent's assumptions are true, I just mean for the sake of argument. In order to counter someone's argument, you must show that either their assumptions are wrong, or that their assumptions don't lead to the conclusion that they're arguing for. In this case, its easier to show that the assumptions don't lead to the conclusion, than it is to show that the assumptions are wrong, so you should argue as though God exists, even if you don't think he does.
My assumption about God in my argument is meant to show that IF God exists, then assisted suicide should be legal. I'm not claiming that God exists, but that the argument that assisted suicide should be illegal because God said that suicide is immoral is invalid.
As for morality, I don't want to turn this into a debate about semantics. I believe that certain things are beneficial, good, or right, and that certain things are harmful, bad, or wrong. Neither the soul nor civil reasons are involved in my concept of right and wrong. I'll admit that I don't have a very strong logical argument for morality, but as I mentioned, I don't think life has any point without my belief, and it is impossible to prove or provide evidence for something being good or bad if you don't establish that certain things are good or bad first.