It all begins with a jest. The essence of comedy is the coming together of opposites, the juxtaposition of incongruous things. So we laugh when an adult speaks like a child or when a simple man finds himself lost amid the complexities of sophisticated society. The central claim of Christianity -- still startling after two thousand years -- is that God became human. The Creator of the cosmos, who transcends any definition or concept, took to himself a nature like ours, becoming one of us. Christianity asserts that the infinite and the finite met, that the eternal and the temporal embraced, that the fashioner of the galaxies and planets became a baby too weak even to raise his head. And to make the humor even more pointed, this incarnation of God was first made manifest not in Rome, Athens, or Babylon, not in a great cultural and political capital, but in Bethlehem of Judea, a tiny outpost in the corner of the Roman Empire. One might laugh derisively at this joke -- as many have over the centuries -- but, as G.K. Chesterton observed, the heart of even the most skeptical person is changed simply for having heard this message. Christian believers up and down the years are those who have laughed with delight at this sacred joke and have never tired of hearing it repeated, whether it is told in the sermons of Augustine, the arguments of Aquinas, the frescoes of Michelangelo, the stained glass of Chartres, the mystical poetry of Teresa of Avila, or the little way of Thérèsa of Lisieux. It has been suggested that the heart of sin is taking oneself too seriously. Perhaps this is why God chose to save us by making us laugh.
Fr. Robert Barron, Catholicism: A Journey to the Heart of the Faith, pp. 9-10