What is propositional knowledge?

A proposition is basically just a claim abuot the world. It can be justified or unjustified; true or false; believed or not believed. For a proposition to count as knowledge, many think that it must be justified true belief. First, it seems obvious to say we cannot know something that is false. I can not know that sky is red, because it is not true that sky is red. Second, it seems that we have to believe a proposition in order to say we know that it is the case. When I say 'I know Mr X is guilty,' I have to believe he is in order to know it. 

Thirdly, it has to be justified in order to count as knowledge. This is because people can have true beliefs. Let's say Mr X did it, so my claim 'Mr X is guilty' is a true belief. But if I'm judging his guilt based on the colour of his socks, it looks like I don't really know that he's guilty. My reasons must therefore matter. So,for a proposotion to count as knowledge, it must be a justified true belief.