There are two main themes of Ephesians:
(1) Christ has reconciled all creation to himself and to God, and
(2) Christ has united people from all nations to himself and to one another in his church.
These great deeds were accomplished through the powerful, sovereign, and free working of the triune God—Father, Son, and Holy Spirit—and are recognized and received by faith alone through his grace.
In light of these great truths, Christians are to lead lives that are a fitting tribute of gratitude to their great Lord.
More about the Ancient City of Ephesus
An important port city on the west coast of Asia, Ephesus boasted the temple of Artemis (one of the Seven Wonders of the ancient world). Just a few decades before Paul, Strabo called Ephesus the greatest emporium in the province of Asia Minor (Geography 12.8.15; cf. 14.1.20–26). However, the silting up of the harbor and the ravages of earthquakes caused the abandonment of the harbor city several centuries later. Today, among the vast archaeological remains, some key structures date from the actual time of the NT.
The grandiose theater, where citizens chanted “great is Artemis of the Ephesians” (Acts 19:29–40), had been enlarged under Claudius near the time when Paul was in the city. It held an estimated 20,000 or more spectators. The theater looked west toward the port. From the theater a processional way led north toward the temple of Artemis. In the fourth century b.c. the Ephesians proudly rebuilt this huge temple with their own funds after a fire, even refusing aid from Alexander the Great. The temple surroundings were deemed an official “refuge” for those fearing vengeance, and they played a central part in the economic prosperity of the city, even acting at times like a bank. A eunuch priest served the goddess Artemis, assisted by virgin women. Today very little remains of that once great temple beyond its foundations and a sizable altar, although the nearby museum displays two large statues of Artemis discovered elsewhere in Ephesus.
The wealth of some residents of Ephesus is apparent in the lavish terrace houses just off Curetes Street. Later inscriptions mention a guild of silversmiths and even give the names of specific silversmiths (cf. Demetrius the silversmith, mentioned in Acts 19:24). However, as in most Roman cities, many people would have been in the servant class, and others would not have claimed much wealth. By the end of the second century (after the NT period) many other monumental structures were added, including some important gymnasia and the famous Library of Celsus. Remains of the giant Byzantine Church of Mary remind one that this former pagan town later hosted an important church council (the Council of Ephesus, a.d. 431).
This information and much more about the letter to the Ephesians can be found at http://www.esvbible.org/resources/esvsb/introduction-to-ephesians/