Fra Angelico, The Story of St Nicholas: Giving Dowry to Three Poor Girls, c. 1437

In order to become a Saint, you not only have to devote your life to God, but you have to do tons of good deeds and usually perform a few miracles too (miracles are semi-magical things that happen when you pray a lot).  He earned his solid reputation as a generous giver through the various kindnesses he performed throughout his life.  In his best-known legend, he aided the three daughters of a poor man.  In the days when fathers paid prospective husbands to marry their daughters, the girls were doomed to never be married and would be sold into slavery or prostitution instead.  To spare them from that fate, Nicholas dropped purses of gold into their window, as he is seen doing in the painting above, providing enough for their dowries and allowing them to be courted.