Explanation:

In this final scene the day before Frank turns 16 he goes out to the pub with Pa Keating. After becoming extremely intoxicated he goes home where he confronts her affair with Laman Griffin.  The conversation escalates and Frank loses control and "I slap her on the cheek so that key so tears jump in her eyes."  This action leads his to question his faith at great lengths; Finally a kind priest confronts Frank and tells him that god has forgiven him for his sins, and that he now needs to forgive himself.
*Quote found on page 341 of Angela's Ashes by Frank McCourt