Since we know that the original audience for the Revelation to John were seven churches in ancient Asia Minor, it should come as no surprise that the book begins by addressing specific issues with each of these seven churches. The letters are highly symbolic and draw on imagery that is unique to each of the cities mentioned (I will attempt to point this out as we go along).
Ephesus was a major affluent cosmopolitan city in the ancient world and was the home of the temple of Artemis. Artemis (or Diana) was a fertility goddess and was worshiped through ritual temple prostitution. The Temple of Artemis was one of the 7 wonders of the ancient world (along with the Pyramids of Giza, the Colossus of Rhodes, etc.). Ephesus was also the center of the gnostic controversy that is likely at the root of the critique of the Ephesian church. They are hard workers, they are patient and enduring. However, they are struggling with false teachers and in the hard work and patient struggle they have "lost their first love" (2:4). It is easy for Christ followers to become so task oriented that we forget that our faith is not about religious observance but about a relationship with Jesus. That relationship is fundamentally one based on love (he first loved us).
The second letter is to Smyrna. This is the only one of the 7 cities that remains today (called Izmir today). This city in its first 300 years was destroyed and rebuilt several times (perhaps the reference to death and resurrection in the opening line is a play on this history). Synagogue of Satan probably refers to the temple of the Emperor in Smyrna. Emperor worship was central to the life of this city and persecution against the Christians, who refused to bow to the emperor or worship at his temple, was often severe.
The third letter is to Pergamum which was the seat of the Roman Governor for this area. The Governor has the power of the sword (hence the reference to Jesus with his two edged sword). Where "Satan's" throne is: is a reference to the governor's palace in this town. The core issue appears to be compromise with the imperial cult (emperor worship) by some Christians. The church must always struggle against any attempt to accommodate the political realities where they compromise the clear teaching of Jesus. The "white stone" mentioned here is a likely reference to a small white stone used to serve as an "admission" ticket to various events and temple rites.
The fourth letter is to Thyatira. Thyatira has no historic significance other than that it was a center of what we would call "trade unions". The difficulty was that each trade had a patron god or goddess and to be part of the trade union (weaving, baskets, ceramics, etc.) one was required to participate in the activities of the appropriate temple. This would involve participating in the "ritual fornication" as well as the following feast. This is why the critical reference here is to the "practice of fornication and to eat food sacrificed to idols" (2:20). The admonition to avoid food sacrificed to idols goes back to Acts 15 and the Jerusalem council. The early gentile Christians were often at a severe economic disadvantage following a faith that requires us to turn away from idols and idolatry while at the same time living in an economic climate where the idols were completely intertwined with the economic life of the community.
The final message for each of these cities is to to turn away from these false practices and behaviors and to return to faithfulness. At the heart of biblical Christianity is a call to return and begin again. It is a call we need to heed on a daily (perhaps even an hour to hour basis). Am I staying with my first love? Am I overcoming the temptations and obstacles of life?