Not seen that film for ages, but if it's the bit I'm thinking of, it's a Christmas carol called "O Holy Night" composed by Adolphe Adam. There are many recorded versions (try Aled Jones to start with). (The main chorus includes the words 'Fall on your knees', if you want to confirm that it's the right song.)
"Carol of the Bells' is also sung at some point in Home Alone. (That is much faster and has the same tune all the way through, passing between the parts. It includes the words 'ding dong ding dong' a lot...)