Was Sardis a fake church?
It was not easy to live the gospel at the end of the first century.
Remember, for true disciples, Jesus said the road was narrow. The way forward would be fraught with many dangers, toils, and snares. John, the last of the surviving disciples, had been living in exile. The church was under persecution. In many places, the persecution was fierce. You had seven letters written to seven churches living in a hostile world in situations that would make or break a church. You can really see what people are made of when they're under pressure. Character is revealed under fire.
With that in mind, let's read Revelation 3:1-6:
And to the angel of the church in Sardis write: "The words of him who has the seven spirits of God and the seven stars. 'I know your works. You have the reputation of being alive, but you are dead. Wake up, and strengthen what remains and is about to die, for I have not found your works complete in the sight of my God. Remember, then, what you received and heard. Keep it, and repent. If you will not wake up, I will come like a thief, and you will not know at what hour I will come against you. Yet you have still a few names in Sardis, people who have not soiled their garments, and they will walk with me in white, for they are worthy. The one who conquers will be clothed thus in white garments, and I will never blot his name out of the book of life. I will confess his name before my Father and before his angels. He who has an ear, let him hear what the Spirit says to the churches.’"