The seven churches of Rev. chapt. 1 - 3 were located in and surrounded the capital city of Asia Minor, the “seat of Satan” in Pergamos (var. Pergamum) (Rev. 2:13.) The “seat of Satan” was a reference to the center of idolatry, as Pergamos practiced sun worship, which originated from Babylonian idolatry, and their priests used the same vestments and title of “pontiff.”
Their people were called “The Temple-keepers of Asia.” They had several gods, the chief of which was Zeus. The last leader of Pergamum gifted his title of Pontifex Maximus to Rome in 133 BC. The rulers and emperors of Rome used the title until 382 AD, when Emperor Gratian refused it as being un-Christian. But, it is interesting to note that it is still used today by the Pope.
Excerpt from Studies in Revelation by Hamption Keathly III, 1997, p. 63:
"Pergamum was very wealthy, the center of emperor worship with many temples devoted to idolatry. This was the place ‘where Satan’s throne is’ (Rev. 2:13). The phrase has been applied to the complex of pagan cults, of Zeus, Athena, Dionysus and Asclepius (Esculapius), established by the Attalid kings, that of Asclepius Soter (the ‘saviour,’ ‘healer’) being of special importance. These cults are illustrative of the religious history of Pergamum, but “Satan’s throne” could be an allusion to emperor worship. This was where the worship of the divine emperor had been made the touchstone of civic loyalty under Domitian.
Here was the magnificent temple of Esculapius, a pagan god whose idol was in the form of a serpent. The inhabitants were known as the chief temple keepers of Asia. When the Babylonian cult of the Magians was driven out of Babylon, they found a haven in Pergamum. "
"The title of the Magian high priest was “Chief Bridge Builder” meaning the one who spans the gap between mortals and Satan and his hosts. In Latin this title was written “Pontifex Maximus,” the title now used by the Pope. This title goes all the way back to Babylon and the beginnings of the mother-child cult under Nimrod of Genesis 10 and his wife Sumerimus. Later, Julius Caesar was elected Pontifex Maximus and when he became Emperor, he became the supreme civil and religious ruler and head of Rome politically and religiously with all the power and functions of the Babylonian pontiff." Source: "The Message to Pergamum" here
From The Seven Golden Candlesticks by Henry Baker Tristram. p. 63
"Like Ephesus, it boasted on its coins and in inscriptions to be a temple-keeper, and like other Asiatic Greeks, we may be certain that the Pergamenes were devoted to a sensuous and licentious worship....The tutelary diety of Pergamu was AEsculapius, the god of healing.....In his honor, a living serpent was kept and fed in the temple, while the serpent-worship was so marked a character of the place, that we find this reptile engraved on many of its coins." Source: here
The seven churches of Asia had to be strong, and fight to resist temptation, to remain faithful to Christ, and not mix with pagan idolatry. That was one of the reasons for the constant encouragement in all of the letters of the apostles / disciples for the people to continue in their belief in the Lord and the gospel of Christ.
Satan's main tool was to deceive the people to turn them away from our heavenly Father, to worship anything / anyone other than our Creator, and places of idol worship were the work of Satan, thus the seat of Satan.