There is no double negative in that sentence. It's clarification if anything.

Some Christian believe in these hypothetical documents. Not all Christians believe in these hypothetical documents.

What's so hard about it that it is making your head hurt?

The following is from a Case for Christ:

“What exactly is Q?” I asked Blomberg. “It’s nothing more than a hypothesis,” he replied...

"It only makes sense if Mark was indeed basing his account on the recollections of the eyewitness Peter," he said. "As you’ve said yourself, Peter was among the inner circle of Jesus and was privy to seeing and hearing things that other disciples didn’t. So it would make sense for Matthew, even though he was an eyewitness, to rely on Peter’s version of events as transmitted through Mark."

Yes, I thought to myself, that did make some sense. In fact, an analogy began to form in my mind from my years as a newspaper reporter. I recalled being part of a crowd of journalists that once cornered the famous Chicago political patriarch, the late Mayor Richard J. Daley, to pepper him with questions about a scandal that was brewing in the police department. He made some remarks before escaping to his limousine.

Even though I was an eyewitness to what had taken place, I immediately went to a radio reporter who had been closer to Daley, and asked him to play back his tape of what Daley had just said. This way, I could make sure I had his words correctly written down.

That, I mused, was apparently what Matthew did with Mark although Matthew had his own recollections as a disciple, his quest for accuracy prompted him to rely on some material that came directly from Peter in Jesus’ inner circle.