The name Sardis means those escaping: it was a dead church.
The city of this name used to be the important and wealthy capital of the kingdom of Lydia, at the time of king Croesus (550 BC), but it suffered considerable decadence after that. It was twice invaded though lack of vigilance. At the time this letter was written, the cult to the Roman emperor was very strong.
The Lord Jesus introduces himself as he who has the seven spirits of God (Isaiah 11:1-2, Mark 1:8), and the seven stars (Revelation 1:20), indicating this supreme command over the church: it is he who gives the Holy Spirit (on the conversion of each believer, operating in the churches through him), and their leaders are accountable to him (Hebrews 13:17).
Although reputed to be alive, Sardis is in fact dead: the leaders and most of the church's members have no spiritual life, although they call themselves Christians and carry out a ritualised form of worship. They are told to wake up and sort themselves out, strengthening what little within them is still alive, because Christ has not found their works perfect (meaning complete) before God. They must remind themselves of how they had originally received and heard the true Gospel of Christ and the sound doctrine of the apostles; they must submit to it with repentance. If nothing is done, the Lord tells them he will come upon them unexpectedly. Even in Sardis, however, there are a few who have not "soiled their clothes"; they have not been tainted by the corruption inside it, but remain true to Christ; these shall enjoy the company of Christ, in purity.
Sardis is a type of the majority of the churches during the Reformation period, from about 1500 AD to 1750 AD; as the Roman Catholic Institution reached abysmal levels of heresy, idolatry and immorality, escaping out of it came Tyndale, Cranmer, Le Févre, Farel, Luther, Calvin, Zwingli and others, giving rise to Protestant institutions which turned again to the Bible and spurned the Roman Catholic heresies.
At the same time, the avarice and tyranny of the popes weighed heavily on the European monarchies, and some of them took advantage of the opportunity to unshackle themselves from the pope's dominion and to give their support to the reformers. They then created established reformed "churches" in their countries, to the exclusion of all others. The persecution of true believers, who denied the spiritual authority of such institutions, continued throughout Europe, to which were now added "religious" wars with political motives between the remaining popish empire and the Protestant kingdoms and principalities. Entire populations were decimated, or forced to flee to escape prison or death for not submitting to the established "church" in their country.
Having denied the authority of the pope, these institutions divested themselves of the idolatry and much of the heresy of Rome, and adopted the Bible as the only rule of faith; they had good solid creeds, and so it appeared that they were living churches, but the work was not complete: they still retained the doctrine of the nicolaitans (clericalism), infant baptism (thereby denying the need for new birth in the Spirit), and were shackled with the secular ruling powers. In fact, they were dead: there was no spiritual life because they were mostly composed by and led by unbelievers. They must repent and revive the little spiritual life existing in them, and return to the true Gospel of Christ and the apostolic teaching which had motivated their separation from Rome. Otherwise Christ will come unexpectedly to them (contrary to the living church, which expects and is prepared for its rapture).
Even during this period, some believers and churches resisted pressure and persecution, and remained untainted; they are assured of the presence of Christ in holiness, for they are worthy.
The message to he who overcomes, is that he will be clothed in white garments (salvation - 7:14, also triumph, purity, glory), and that his name will not be blotted out of the Book of Life (he will never lose his salvation, even though he may be "excommunicated" by the ecclesiastic authority in his country!); the Lord Jesus will declare him to be his own, before His Father and his angels.
1 "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, that you have a name that you are alive, but you are dead.
2 "Be watchful, and strengthen the things which remain, that are ready to die, for I have not found your works perfect before God.
3 "Remember therefore how you have received and heard; hold fast and repent. Therefore if you will not watch, I will come upon you as a thief, and you will not know what hour I will come upon you.
4 "You have a few names even in Sardis who have not defiled their garments; and they shall walk with Me in white, for they are worthy.
5 "He who overcomes shall be clothed in white garments, and I will not blot out his name from the Book of Life; but I will confess his name before My Father and before His angels.
6 "He who has an ear, let him hear what the Spirit says to the churches." '
Rev 3:1-6 (NKJV)