"early Christians merely went with the existing custom."
Not exactly. Early on, the church outlawed Christmas. Too much debauchery. After a while the wild partying got toned down a bit. Someone penned some pretty carols, some people started feeling all cheerful and generous of heart. The festivities helped brighten the dark and cold winters. The story of the babe in swaddling clothes spread, and the Church really latched on to Dec 25 being Jesus' birthday and the whole "reason for the season" movement was born (again).