Write what you know. We've all heard that often enough. But what does it mean? Does it mean I need to be an expert about something before I write something? Does it mean I can only write about what I know to be a fact if I expect to find success as a writer?
I have always taken it to mean that I should show conviction in my writing, regardless of the topic.
It does not command me to write only what I know, but to write as if I know it. In other words, I should take the spirit of the words and messages I am trying to convey into myself; I should own them.
Write what you know then becomes write what you believe. Be honest with your readers. Don't attempt to deceive them, unless you are actively trying to deceive them (that's a different story) but instead be forthright in what you claim in your story.
If you are writing fiction and your main character is faced with a tiger or a lady or a speeding train, be prepared to account for their actions as they fight the tiger, escape the train and rescue the lady. If you cannot believe what you are writing the chances are that your readers won't believe it either.
I can hear you already: "But what if I'm just writing from the heart? What if I don't 'know' it but only feel it? How do I do that?"
Well, when you are writing your own personal beliefs (religious views, political views, issues) you are already writing what you know because you are sharing your personal beliefs, the things you know in your heart are true. No one can argue that you don't feel a certain way, if you indeed feel that way. Be certain you do not present what you believe as fact, except as it pertains to the way you feel. In other words, don't say "Writers suck." You can say, "I believe writers suck because..." That is a much more honest way of presenting your thoughts.
Yes, yes, you are writing because you want to convince someone to lean in your direction-you want to influence their attitude. That's great and it is what a good many of us writers aspire to. The rule is the same however: honesty is the best policy.
Write what you know. If your point is worthy of their attention, if it inspires them to change their opinions about a certain issue, that is a testament to your ability as a writer to accurately convey your point of view to such an extent that you are able to sway the hearts and minds of others.
Write what you know. And if you don't know it, at least believe it. Convictions carry a lot of weight.