The city of Sardis and its surrounding area were watered by the river Pactolus. The river, known for its golden sands, helped make the city prosperous when gold was found near its banks. The city was also noted for its fruits, wool, and temple to the pagan goddess Cybele (whose worship was very similar to the worship of the pagan goddess Diana (Artemis) found at Ephesus).
Sardis was the ancient capital of the Kingdom of Lydia. The kingdom's most noteworthy king was the very wealthy Croesus (Kroisos), who ruled from about 560 to 547 B.C.
Croesus was the first person to strike and issue the first true pure gold (and silver) coins used in the marketplace. His father King Alyattes, who reigned from about 610 to 560 B.C., minted and distributed the world's first coins. Alyattes' coins were made of electrum, an alloy of gold and silver.
The city became important to the Persians after it was conquered by Cyrus the Great in the 6th century B.C. By the first century A.D. it had passed into the hands of the Romans. Although an earthquake destroyed it in 17 A.D. the city was soon rebuilt.
The church at Sardis had a number of faults, although some were faithful. They were spiritually dead overall, despite having a reputation of being spiritually alive (Revelation 3:2). Jesus, in his evaluation of them, found them deficient in regard to doing enough good works (verse 3).
Those in Sardis desperately needed to spiritually wake up and repent. They had the knowledge of the truth, but were not practicing it, since they needed to do what they had heard and received. Jesus warned them to wake up and repent before He returned.
Some, however, in Sardis were faithful and had not soiled themselves spiritually (verse 4). They will walk with Christ since they will overcome. Their names will not be removed from the book of life and the Lord will acknowledge them before the Father.
In general, a majority of the people in the church at Sardis were spiritually weak, much like the members of the Laodicean church described later in the same chapter. Only a tiny minority of the church were committed and converted Christians. Jesus still wanted the weak members to repent, just like He did with the members of the Laodicean church.
Tradition states that Sardis was the first in its area to be converted by the preaching of the apostle John. It also may have been the first city that revolted from Christianity and one of the first that was laid in its ruins. The inhabitants were dimly viewed by their contemporaries for their voluptuous way of life. There may be an allusion to this fact in God's message to the church found in the book of Revelation.
And to the angel of the church in Sardis, write: These things says He Who has the seven Spirits of God and the seven stars.
I know your works, and that you have a name as if you are alive, but are dead. Be watchful, and strengthen the things that remain, which are about to die. For I have not found your works complete before God.
Therefore, remember what you have received and heard, and hold on to this, and repent. Now then, if you will not watch, I will come upon you as a thief, and you shall by no means know what hour I will come upon you . . .
The one who overcomes (especially in Sardis) shall be clothed in white garments (symbolic of being spiritually clean); and I will not blot out his name from the book of life . . . (Revelation 3:1 - 3, 5, HBFV).