There is a profound difference between knowledge and certainty. If one is certain that a particular proposition is true, he is fully confident that it is true; yet theoretically he could be right, or wrong. If he is right, then he knows that it is true; his certainty is knowledge. If he is wrong, then he mistakes that it is true; his certainty is a mistake.
Remember that there are many truth claims that contradict each other, only one of which is true. For example, consider the identity of Jesus. Is He God? Is he a semi-god? Is he a mere man, but a good teacher nonetheless, whose life and message are exaggerated in the gospel accounts? A sincere lunitic? A profoundly wicked liar who got what was coming to him? Or perhaps he never existed in the first place. All of these are exclusive statements about Jesus' identity. Only one of them is true. Every position has its adherants, each of whom is certain about his position. Yet to not only be certain of but actually know the truth, one has to be correctly certain, by being certain of the correct proposition of Jesus' identity.
Now, of course, one may choose not to be certain on any of these positions, but simply affirm that he doesn't know. After all, if he isn't certain which proposition of Jesus identity is correct, then he can't be mistaken.
Technically, this is true. However, when one takes this course, he is immediately confronted with another set of conflicting propositions. This time, it is more simple, for there are only two:
1. If one doesn't know Jesus' true identity, then the consequences aren't seriously bad.
2. If one doesn't know Jesus' true identity, then the consequences are seriously bad.
One cannot escape the issue. He must now either affirm the first proposition and rest content in his refusal to take sides, or affirm the second choice and do everything he can to find out the truth of Jesus' identity.
This same principle applies to any issue, really. If one refuses to take a side in a debate, but instead be contentedly agnostic about it, he is simply affirming his certainty that the debate doesn't matter and he doesn't need to know which side is true. Yet he still passes judgment on the issue.
We can conclude, then, that although it is possible (and all to common) to be mistaken, it is impossible to escape being certain about an issue when one is confronted with it.