Ephesus is a "must see" for travellers with an interest in early Christian history and sites because both St. John and St. Paul lived and preached there, albeit at different times, and St. John died and was buried nearby.
St. John introduced Christianity to the city and was leader of the Christian community there until his exile to Patmos, from whence he returned some time after writing the Book of the Revelation. In the meantime, as we can read in the Acts of the Apostles ch 19, v1 "while Apollos was at Corinth, Paul took the road through the interior and arrived at Ephesus".
By settling in the city St. Paul fulfilled a promise he had made during a brief visit from Corinth, to return. Paul preached all around the city, in the markets, the Great Theatre and the Temple of Artemis, quickly gaining converts as he preached. Soon he had a large following or "church" which became the senior of the seven churches of Asia Minor and the third largest church in Christendom after those of Jerusalem and Antioch. Paul stayed in Ephesus for some 3½ years, during part of which time he was imprisoned and probably wrote his "Epistles to the Ephesians".
Whilst he was making his converts, St. Paul was opposed by state officialdom, magicians, fortune tellers and devout followers of the Goddess Artemis. Many jewellers and silversmiths in the city derived a large part of their income from the manufacture and sale of miniature images of the Goddess. Their leader, Demetrius, and some of his colleagues stirred up a mob of thousands and led them to the Great Theatre chanting "Great is Artemis of the Ephesians".
Paul was prevented by his disciples from facing the mob, which eventually dispersed after the City Clerk said that anyone with a complaint should have it dealt with by the Court. St. Paul left the city after this event and leadership of the church was re-assumed by St. John on his return from exile.