The person we call Jesus did exist. At the synod of Nicea (325 AD from memory) Constantine I called all the christian leaders from around the empire to agree upon a standard form of christianity from the disparate forms that had arisen in the 300 odd years since the passing of Jesus. Many of the christian holy events were placed on or near old pagan festivals and pagan holy places to help the spread of christianity. Christmas is on/near the festival of Saturnalia. Many early christian churches were built on old pagan sites . At Avebury (UK), there is a church built not only within an ancient stone circle, but it is built from some of the giant stones itself. Think of it as continuity of worship for new converts to christianity (in antiquity)