It's hard to believe how long it's been since I last saw this film. We finally got a DVD of it last year but didn't watch it then. This is one of my favorite Christmas movies. I can't find a trailer for it that can be embedded (though imdb has it online).
Here's a short clip:
Variety calls it "warmly sentimental". TCM has an overview. Time praises Bergman:
As Sister Benedict, Ingrid Bergman manages to combine beauty, great good humor and saintly dignity even while swinging a baseball bat. Taking her role seriously, Actress Bergman played it without make-up (with no damage to her good looks), visited parochial schools to see how nuns actually behave, wore ballet slippers under her robes to perfect a gliding step.