The winter solstice has been a key religious date, most likely since pre-history, because it's the darkest part of the annual cycle. The Church chose to memorialize Jesus' birthday at that time in order to compete with the Roman saturnalia festival around the same time and appropriate its use of lights and other symbols. One of the geniuses of the Christian Church has been to borrow and 'christianize' primaeval stuff.
Better evidence for god-as-metaphor I cannot imagine. Maybe, instead of christianizing primitive religions, they were simply doing what all religions before them had done - making their own cultural metaphors about nature.
Most scholars believe they adopted the winter soltice as the birth date of Christ in order to replace the pagan rituals and help everyone move towards Christianity.