I would highly recommend no getting a free theme. You can get premium themes on themeforest.com for $40-60, and they will look professional as opposed to free. If you are interested in some specific ones, I'd be happy to point you to a few.
Incarnation is a great theme, with built-in functionality for sermons. I've used it before, and it does a bit of customization to replicate what's on the demo (and you need pictures that will fit the slideshow on the home page), but it's very professional looking and has good functionality for a church.
I haven't used it, but Adore seems to be a good one.
Rock Harbor also looks good (and would look better with a good church behind it instead of the one on the demo).
Tim Challies did the design for Westminster Letter Press back in 2009; he does good work and free is a whole lot less than what I paid! Westminster Letter Press