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."
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.