I do believe

I do believe

I believe that what we're discussing is true. "Do" here is used for emphasis. I do believe Judy is coming—she would have texted me otherwise. A: "Thomas and Millie got engaged, I do believe." B: "Yep, it's true—I saw her ring today."
See also: believe
Farlex Dictionary of Idioms.

I do believe.

a way of affirming or reaffirming one's opinion. Jim's in love with that gal, I do believe. Jane's store will do well, I do believe.
See also: believe
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • don't you believe it
  • don't you believe it!
  • believe you me
  • Believe you me!
  • believe me
  • believe one's ears
  • believe (one's) ears
  • believe ears
  • so (something) it isn't true
  • so bad, stupid, etc. it isn't true