I do not know how to answer it. The birth of each our children came to me a miracle resulting from an act of love, but that doesn't fully explain it. It's more than just me and my wife and our love. The birth of our children shows me the glory and the mystery of life. It makes me want to believe in God.
Birth is the problem for the atheist.
But in Christianity the symbol is fact. This night the child which we envision, and to whom we say happy birthday stands as the sign for us as for the shepherds, that the needs of our hearts and our minds and our souls, our great spiritual remiss and longings can be resolved in new life, in the birth of Jesus in our lives. God's gifts are never given like presents in a Christmas stocking. They are always challenges that we must take up. And yet, it is not we who start anything.