Prester John was the greatest Christian King who never lived. All through the Middle Ages there were persistent legends (sometimes backed up by apparent documentation) about this powerful priest-king in the East who was ready to bring his powerful armies to attack the Muslims in concert with the Western Crusaders.
The Book of Common Prayer was written during the English Reformation as the new reference for the services & ceremonies for the new Church of England, and it's still in use in many churches today. Discussing it on In Our Time were Diarmaid MacCulloch (University of Oxford), Alexandra Walsham (University of Cambridge) and Martin Palmer (Director of the International Consultancy on Religion, Education, and Culture).
Gnosticism was part of the growth of secret knowledge cults in the first few centuries AD, flourishing in the 2nd & 3rd Centuries. Although not necessarily associated with Christianity it is best known as a different interpretation of Christianity, and the mainstream Christian Church reacted against what they regarded as heresy in ways that are still part of Christianity today.