The Gospel of John is a paradoxical book to me. John's tone (both in the Gospel and his letters) is very matter-of-fact and this leads to a lot of quotable verses (of course the most famous of all being John 3:16). But I think that there is a fundamental problem with this because John's books are so context sensitive (there are broad ideas being communicated and reading individual verses, though they may seem to pass along concrete ideas, are often really slaved to the main idea of the passage that can span entire chapters). So this is central to the interpretation of the book and always has to be kept in mind - that it often sounds like John is quoting proverbs, but they link together to form an entire idea. A couple of verses will show what I am talking about here, I think:
John 5:31 - "If I testify about myself, my testimony is not valid."
John 8:14 - Jesus answered, "Even if I testify on my own behalf, my testimony is valid, for I know where I came from and where I am going."
John's tone makes us want to take each statement as simple standalone fact because they are so straightforward. But if both of the above separated statements were meant to be true by themselves then Jesus contradicts himself. An atheist may have no problem with this, but I'm not an atheist, so I choose to believe that this is not the case and rather we must understand each passage in the light of the entire passage, not in individual verses pulled from their context. In the first case Jesus wants to point out that there is another who testifies about him (John 5:37 - "And the Father who sent me has himself testified concerning me.") and that this is not just anybody, but rather is the Father himself. In John 8 Jesus has a similar concern (to point out that his testimony is given validity not because of himself, but rather because God the Father testifies concerning him). We can see this in John 8:18 where Jesus says, "I am one who testifies for myself; my other witness is the Father, who sent me."
Of course, I've just quoted other verses in the same area to point out that I don't think we should be quoting individual verses, which is a little silly of me, but I think that examination of the entire passage in each case proves the point.