The fact that you believe what you're saying is not enough. Not if you want to be convincing.
Show your conviction
You must allow that belief to show in your voice and body - whatever the medium: television, radio, print or internet. Yes, even when you talk to a print reporter on the phone, subtle signals go down that line that affect the reporter’s reaction to you and the topic, affecting the way the article gets written. Just getting the facts correct will usually not carry your argument in any speaking situation.
How do I show my conviction?
A little extra energy in your voice, a little more emphasis, more animation. Something in your tone that says, I'm keen that you get this.
Many centuries later, Cicero’s words still hold. “What convinces is conviction. You simply must believe the argument you are advancing, otherwise you are lost. No chain of reasoning, no matter how logical or elegant or brilliant, will win the case, if your audience senses that belief is missing.”
Reporters may tell you otherwise, but the sad truth is that many who do honestly believe their own argument, inhibit their own conviction and so lose credibility.