I should hope you believe what you say! If you don't believe what you say, either:

1. Don't write it!
2. Qualify that you're echoing someone else's words and are setting up a straw man ("Some experts say that... but my research shows...).
3. If this is a new belief for you, say "I have recently learned that" or "I have just realized that", and help us understand why we care.

Whatever you do, just don't start your sentence with "I believe that..." It's a waste of three words and makes you sound like you're not prepared to make decisive statements.