First of all, I try to avoid questions like: Do we really know anything for certain? and Isn't it all just opinion? I think those questions slow down genuine thought and are simply questions badly phrased. Sure, you could make an argument that would state: Because of ideas like "The earth is flat" and "Fire is an element" and other debunked theories, you could say that everything is a theory waiting to be proved wrong. However if this were true, would you have any confidence in anything at all? Ask yourself, what do you know? What is this knowledge based on? Usually, knowledge is based on evidenciary support. But, since you are questioning if we know anything. Evidence must then not be fullproof either. So, now what is your knowledge based on?

Probability.

What do the masses tend to think? What do signs point to? Both of these questions are a factor in probability. People believed the earth was round because everyone did, and they didn't notice the signs that pointed to other theories. This is not an impenetrable idea, probability. There are things that just don't fit. But then again there are things we really just don't know... So, we don't know NOTHING, but we certainly don't know everything.