The letters to Thyatira and Sardis are read as the fourth and fifth church ages. To follow them we need to trace what happened: how God's word was carried forward while the power of Rome grew beside it.
Thyatira (about 600–1500): the faithful few
To Thyatira Christ comes as the Son of God, with eyes like a flame of fire and feet like fine brass: a stern, searching gaze, and feet firm and steady. Yet he first praises their works, love, faith, service and patience.
That praise belongs to a line of faithful men. Martin, a soldier from the region of today's Hungary, believed and within a year or two left the army to preach. He settled at Tours in France and began a monastery. Finnian, an Irishman, studied there, took Jerome's Latin translation of the Scriptures to Ireland and taught many; twelve of these went to Scotland. One, Columba (Colum Cille), who went about the year 600, is counted as this age's messenger. He travelled to Tours for the original gospels, probably in Greek, and took them to Scotland to teach from them.
Rome's rise
Meanwhile the bishop of Rome was gaining power. Living close to the emperor, he was drawn into politics, and his office grew. By about 600 the empire had split into a West, ruled from Rome, and an East, ruled from Constantinople, the city Constantine founded. Invaders broke the West into pieces, and the East grew stronger. Around 600 an eastern emperor, Phocas, recognised the bishop of Rome as head over all bishops and churches. From the time of Gregory and Boniface, the office of pope existed.
Popes and emperors then propped each other up. The pope recognised Charlemagne, king of the Franks, as Holy Roman Emperor, and later the German Henrys, while an emperor who disliked a pope removed him and set up another. The king appointed the pope and the pope the king, from 600 to about 1500.
This is why Revelation speaks of seven heads and an eighth that is of the seven: the emperors kept political authority, but Phocas handed religious authority to the pope, as though one head had become two.
Jezebel: the Lord's charge
Christ charges the messenger: you allow that woman Jezebel, who calls herself a prophetess and leads his servants into immorality and into eating food offered to idols. Thyatira had no woman by that name, but someone of her kind, and the Lord applies the picture to the age.
Beside the faithful few a great church formed, headed by a pope whom the king chose. Politics decided matters, other religions were mixed in, and images were made of Jesus, the infant Jesus, Mary, Joseph, Peter and other saints. The teaching that three persons together make one God, instead of the one God, arose around the year 300 and was being taught by 600. In this the Lord sees idolatry and a forsaking of God. The messenger is blamed for allowing it: he should have rebuked Rome but said nothing, since Rome had the king's support.
And I gave her space to repent of her fornication; and she repented not.
— Revelation 2:21
The space given was nearly a thousand years, from 600 to 1500. The Lord has since waited four or five hundred years more, and the time of the great tribulation is now close. In it he will cast that church into great affliction and kill her children, the smaller churches that have come out of her and teach other doctrines as the original gospel.
The rest at Thyatira
To the rest in Thyatira, who do not hold this teaching or know the so-called depths of Satan, the Lord lays no other burden: hold fast what you have until I come. These believers taught that God is one, that Jesus Christ is the Son of God and that God came in the flesh. The Trinity teaching confused them, since no one can explain it and the word is not even in the Bible. The Lord asks no deep philosophy, only that they believe Jesus Christ is God and walk accordingly.
Because the church and kings forced their decisions on them, he promises the overcomer strong authority over the nations, to rule with a rod of iron.
Sardis (from about 1500): wake up
By 1500 the Roman world lay in pieces, yet all held one religion headed by the pope in Rome. The teaching had worn away generation by generation, until almost nothing was left. People knew only that Jesus Christ is God; beyond praying, they had no way of knowing what to do or not to do. There was no printing, few could have even a handwritten copy, and copies were kept in a church or in Rome. So the Lord says: you have a name that you live, and are dead.
Then Martin Luther, a priest in Germany, went to Rome and read the manuscripts of the gospels and letters kept there, which Irenaeus had gathered. He took notes, returned to Germany and began to teach: this was the waking up, and the strengthening of what was ready to die. He fixed a long list drawn from Scripture to the church door. He wrote to the pope against the letters of pardon (indulgences), saying that they were useless, that money should not be collected, and that people should be taught to rely on faith in God. Columba should have opposed Rome in his day and did not; Luther did, and from his protest came the name Protestants.
The Lord warns that if Sardis does not wake, he will come as a thief. That coming belongs to the Laodicean age, our own, when his return is near. Had Sardis not awakened, the word would not have reached people, and they would have been in darkness when he came. The few who followed Luther and received the word through his research have not defiled their garments. They will walk with Christ in white, and he will not blot out their names from the book of life. After a thousand years of decline, Scripture was searched out and taught again, like life returning to the dead.
Takeaway
Hold fast what you have understood until the Lord comes, and walk by it. Stay awake, because the word must reach people, and those who sleep will be overtaken like a thief.