If that sounds like a pretty disingenuous way to go about finding the truth, then you're right. However, if you're an atheist like me, the presuppositionist can turn it around on you and say that you're starting with the assumption that God does NOT exist and that Christianity is NOT true. So, basically Christians and atheists are in the same boat when it comes to proving what's what.
There's just one big problem - such an argument is pure and total horse crap.
It can be broken down really simply if I just start with myself. For me, my quest to figure out what's real and what isn't began with the presupposition that there WAS a God. Many other atheists began that way as well. In other words, we began with the presupposition but were unable to make the conclusions fit. There are plenty of examples, some of them were heavily into the church and were even pastors.
But what about people who start out as atheists and become Christian? Does that put the two positions on equal footing yet again? No, it doesn't, because if one can presuppose God and arrive at atheism, it doesn't matter if one can go the other way around. What matters is that the presupposition was irrelevant if it can get you to either conclusion.
Also, a person's unwillingness to use another's presupposition does not equate to them making their own presupposition. Personally, I don't start with the notion that atheism is true, despite the fact that I get accused of doing so. The only presupposition I make is that there should be sufficient evidence for something before we say that it is true. From my understanding, the God question has failed to meet the burden of proof. I'm not assuming it's true that a God doesn't exist, I'm CONCLUDING it, and my conclusion is subject to change as new evidence comes to me.
So, let's stop trying to equate the presupposition of the existence of something is the same as rejecting that presupposition. Let's just examine the evidence and follow it to wherever it takes us, even if it takes us down a path that proves us wrong.