With the departure of the Roman legions, Britannia island was invaded by several germanic peoples. Among them, the Saxons, the Angles (we say Anglo-Saxon), the Jutes, the Danes… Those peoples were pagans, not Christian. As a result, Christianity, what we call the Celtic Church, declined. But it maintained its existence. That Celtic Church, later on, will be absorbed into the Church of the Angles started by St. Augustine of Canterbury. The Church of the Angles, in Latin Ecclesia Anglicana, is what we call today the Anglican Church.