It is as important as what you say.
Your words, inflection, demeanor, and volume communicate something about which you speak.
When you talk about Jesus, are you boring, dry, and uninteresting?
Are you angry, frustrated, and mean when you talk to a family member about your faith?
Do your words about being a believer come with energy and enthusiasm?
Are you happy when you talk about being a Christian and part of a Church?
Your mood will carry a message, no matter what words you use. So make sure the emotions fit the topic when you are talking about Jesus.