This is one of my favorite Christmas movies. I'm not sure why, because I don't tend to like sentimental movies, and this is definitely sentimental.
Variety calls it "warmly sentimental". TCM has an overview. Rotten Tomatoes gives it a score of 94%. 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.