Your normal voice is the voice you use in normal situations… and preaching/mcing is not a normal situation. It’s a very strange situation. You’re infront of hundreds of people. How would you speak if you didn’t have a microphone? Loud? Overly animated? Slower? Yes! That’s your normal voice… for that situation.
So why have a microphone? A microphone now gives you more ranges, volumes, options. Now the situation is: In front of a large crowd, with a microphone. So speak as the situation requires.
Speaking out the front requires more animated, energised vocals and attitude. The microphone doesn’t amplify your personality, just your sound waves.