How often have you been absolutely sure that you are right and it later showed that you were wrong? Never? You cannot remember? Then there are three possibilities: 1. You are never absolutely sure. 2. You have a bad memory. 3. You are lying. You remember that it happened some times? Very good. And how do you explain that you still can be absolutely sure of things? Because everything speaks in favour of them? Was that not the case when you were sure of something that later showed wrong? If you have an argument with another person and both of you are sure to be right even though your opinions are contrary: What makes you be sure that you and not the other one is right? Because his arguments make much less sense? They might make less sense for you, because they do not fit into the other opinions and knowledge that you have. But for the person opposite it might be exactly the contrary. Your arguments do not fit as well into his outlook and experience as his arguments do. So for you everything (that is everything you know) speaks in favour of your views, but for somebody else nothing might speak in favour of these views. And this not only can happen with difficult questions for example about religious beliefs, but also with very simple experiences. People often experience the same situation very differently depending on their background. This is one of the main reasons why there are so many misunderstandings in human communication.
Even if everybody has the same opinion, that does not mean that this opinion is absolutely right. There was a time when everybody thought that the earth was flat. And even if you deny that the people at that time were real scientists, you can hardly deny that Isaac Newton and his colleagues were real scientists. But they all were proven wrong more than 200 years later by Albert Einstein. Because of these experiences that are typical for the history of science and because of logical problems with the verification of scientific laws, Karl Popper developed his now predominating view that it is not possible to verify a hypothesis about nature, but only to falsify it1. So every theory is only temporarily right as long as it has not been falsified2. No scientific "knowledge" is absolutely sure, because the only thing we can say is that it has not yet been falsified. But we can never be sure that it will never be falsified. Already René Descartes realized3 that the only thing that we can be absolutely sure of is that we (as a thinking subject) exist: "I think, so I exist."
Of course there are differences between a new scientific theory which is very specific and general knowledge that can be used every day. The latter has a much higher probability for being correct (i.e. for not getting falsified), because it has already withstood falsification very often and for a long time. But that does not mean that it can never be falsified. So theoretically, there is no real "knowledge" but everything is beliefs with different probabilities. Practically however, we can and do use the term "knowledge" for beliefs that have a very high probability for not getting falsified (e.g. that an object which has more mass than the displaced air falls to the ground when released) and we act as if we were sure that they are correct. As life is finite, there is no problem with this, because it is very improbable that we will witness an apple flying away when we let it fall. But still, only that it has never happened does not mean that it is totally impossible.
If there are different theories that all have not yet been falsified, in which one should we believe? Well, think again about the reason why you might be of the opinion that your arguments are better than the ones of another person. They make more sense in combination with the other ideas that form your outlook on the world and your knowledge. So we believe in those theories, that make the most sense for us, which means that they do not get in conflict with other knowledge, they help us understand things better or they can be used for practical purposes like prediction or production. Those theories are good theories. Because outlooks and knowledge of humans can be very different, the opinion which of some conflicting theories is the best one does not have to be undivided.