Ephesus was the capital and premier city of Asia Minor's western region (known as proconsular Asia). It was the Athenians who initially colonized the city, which is located near the mouth of the Cayster river (and opposite the island of Samos). It was famous for possessing a Temple dedicated to a pagan goddess named Diana. This pagan shine in Ephesus is generally considered one of the seven wonders of the ancient world.
Ephesus, for many years during the Roman period, was considered the second largest populated area of the Roman Empire (the largest being the empire's capital city of Rome). In the first century B.C. the city had a population of more than 250,000, making Ephesus the second largest city in the known world.
Although the apostle John spent a great deal of time in the city, it as Paul who begun the first Christian church within it.
The Apostle Paul, after staying in Corinth for a year and a half, travels to Ephesus in the spring of 52 A.D. The journey to the city, with friends Priscilla and Aquila, is considered part of what is called Paul's second evangelistic or missionary journey. Although he (likely) starts the first Christian church in the city he stays for only a short period.
And after Paul had remained there many days, he took leave of the brethren and sailed away to Syria, and with him Priscilla and Aquila. Now Paul had shorn his head in Cenchrea because he had made a vow. And he came to Ephesus, and left them there; but he himself went into the synagogue and reasoned with the Jews.
And when they asked him to remain with them for a longer time, he did not consent, But took leave of them, saying, "I must by all means keep the feast that is coming at Jerusalem; but I will return again to you, God willing . . . " (Acts 18:18 - 21, HBFV)
During Paul's third missionary journey he returns to Ephesus and lives in it for almost 3 1/2 years due to the success he experiences. Shortly after arriving in the city Paul runs across some believers who, though baptized to repentance by John the Baptist, never received or even knew about God's spirit. He rectifies the situation by rebaptizing the twelve men and laying hands of them in order for them to finally receive the spirit.
Now it came to pass that while Apollos was in Corinth, Paul traveled through the upper parts and came to Ephesus; and when he found certain disciples, He said to them, "Did you receive the Holy Spirit after you believed?" And they said to him, "We have not even heard that there is a Holy Spirit." Then he said to them, "Unto what, then, were you baptized?" And they said, "Unto the baptism of John."
And Paul said, "John truly baptized with a baptism unto repentance, saying to the people that they should believe in Him Who was coming after him - that is, in Jesus, the Christ." And after hearing this, they were baptized into the name of the Lord Jesus. Now when Paul laid his hands on them, the Holy Spirit came upon them, and they spoke with other languages and prophesied (Acts 19: 1 - 6, HBFV).
Ephesus is also the first of seven churches residing in Asia Minor given a spiritual evaluation directly from God. The city is also believed to be the place where the apostle John lived after he wrote the book of Revelation. Tradition states that John died and is buried in the city.