The FAQ, already in the application, addresses this very request directly:
2. My device has a pen. Why can’t I just draw on the screen?
This is a really cool feature idea, but, at least at present, it doesn’t appear to be feasible to implement because of the way text reflows in the application. Doing something as simple as adjusting font size or changing the snap or window size for Gospel Library would cause notes created by a pen to become distorted or out of place. We are, however, working on ideas that will still make the pen a valuable tool for study.
You can try out said experimental pen features with Preview Features enabled in the beta version and Live Pen turned on.