Yesterday's painting of Monica and Augustine illustrates a discussion between Monica and Augustine about a vision he had. Augustine saw a vision of an angelic figure telling him not to worry, one day she (his mother) would come over to his way of thinking, e.g. the pagan philosopher's book he was reading placidly. Augustine couldn't resist the temptation to reveal this to his mother, who scornfully told Augustine that he had it all wrong, the angelic revelation meant that one day he (Augustine) would come over to her (Monica) way of thinking, in fact predicting his eventual conversion. That is why Monica looks so stern and angry with her son.
Augustine relates the incident in Book 3 of his Confessions.