That is disastrous to persuasion. If your listener or reader does not think you firmly believe in what you're saying, he'll automatically question either your sincerity or the quality of your ideas and evidence. There are several ways. Third, and too often by-passed, speak in the active voice, rather than the passive. If you are going to do a lot of verbal communication, this speaking practice will be very valuable.
People read a lot into voices.
You can and should develop this sound if you will be doing much speaking, especially public speaking. And finally, one of the best ways of conveying certainty is to practice speaking up. Make it a habit of not letting attacks on liberty slip by. If you disagree with someone, say so. Know whom you are addressing.