The Magi, or the three wise men who come and kneel before the infant Jesus are a symbol of humility. These three kings come from afar to pay homage to a child born in a stable. They come to worship him as he lay in a manger filled with straw and surrounded by animals.
Their wisdom transcends the poverty of the infant child before them, the Magi, dressed in fine robes, feel honored to stand in his sight. They bring him gifts of gold, frankincense and myrrh, all very valuable, but these gifts do not match the magnitude of the birth of the Savior and the Magi know this.
The Magi, like Jim and Delia, are symbols of humility and humbleness of spirit. They sacrifice their most prized possessions to obtain money to buy a special Christmas gift for the person they love most in the world.
They want to honor each other by showing their love through gifts chosen with great care, just like the Magi did when they came to the baby Jesus's side to witness the miracle of his birth.
"Magi" is another name for the "Three Wisemen" who brought gifts to the Christ child. Aside from the biblical allusion and implications thereof, by calling Jim and Della "Magi", O Henry is commenting on the true spirit of Christmas, that it's better to give than to receive. Both Jim and Della care more about the happiness of their loved one, so they sacrifice their most prized posession.