To the angel of the church in Sardis write: These are the words of him who holds the seven spirits of God and the seven stars. I know your deeds; you have a reputation for being alive, but you are dead. Wake up! Strengthen what remains and is about to die, for I have found your deeds unfinished in the sight of my God. Remember, therefore, what you have received and heard; hold it fast and repent. But if you do not wake up, I will come like a thief, and you will not know at what time I will come to you.
Yet you have a few people in Sardis who have not soiled their clothes. They will walk with me, dressed in white, for they are worthy. The one who is victorious will, like them, be dressed in white. I will never blot out the name of that person from the book of life, but will acknowledge that name before my Father and his angels. Whoever has ears, let them hear what the spirit says to the churches.
~ Revelation 3:1-6
In its time, Sardis was a city well-known for its fame and wealth. Its church members were devout in their outward acts of faith, but inward they were sleeping dead, so much so as to be known as the "sleeping church." So while they may do good deeds and profess devotion to Christ, many members of the church were hypocrites, not truly believing in what they were doing. They lived without the love for or desire to know Christ. So while they were well known for their good deeds, in the eyes of the Lord they were deemed unfinished, lacking the love of God in their works.
And this is the problem with hypocrisy: it nullifies any salvation we thought we may have had. James 1:26 says that if we consider ourselves religious but do not remain spiritually pure, that we deceive ourselves, making out religion worthless. Jesus confirms this in Matthew 6:1, "Be careful not to practice your righteousness in front of others to be seen by them. If you do, you will have no reward from your Father in heaven." To put it simply, we we do deeds to be seen, then we have missed the entire point of the Gospel, which is to love God, and to love others as much as yourself. Living in hypocrisy is only pleasing to the flesh, for in doing deeds to be noticed, you are reaping the rewards of your deeds when it should be God, not you, who is glorified in your works.
In the letter, the punishment seems to be much more severe than the punishment of the other letters. Jesus says he will come like a thief, and your name might be blotted out from the book of life. Many people would choose to look at these verses as proof that salvation can be lost after it has been obtained, but this is not true at all. This is a topic for another discussion, but what these verses really mean is that Jesus will come and take your life as you live it on earth. For there can be two meanings to the book of life: one is the book that has written in it the names of those who have received salvation, and the other is a register of those who are currently living on earth. In the context of this letter, the latter is what is meant here. Jesus tells us that he may come like a thief, and we may have no idea when he will come to take our life, thus when we die our names are removed from the record of those currently living on earth. He says this to accomplish two things: first to scare the hypocrites into reevaluating their lives, and second to give praise to those who are in good standing with the Lord.
To those who are living in hypocrisy, Jesus wants to scare them. He wants people to know that their lives on this earth are temporary, and that we should never have the mindset that we may repent as we are dying, because we may never get the chance. We might be murdered, or get into a some accident, and never have time to repent before we are gone. We will never know when we die, so we should prepare every day spiritually as if we could die that day. So you must ask yourself that question, if you were to die today, how sure are you that you would go to Heaven? And a way to check your answer is to ask yourself this question: if it were just you and God in this world, would you be content? Are you truly pleased with God and the things he is doing in your life, or are you just saying you are to receive the rewards your friends and peers give you for your proclaimed faith? If you are truly worthy, then you should be more than just content with only God, you should be joyful that you get to live with only God and free from the distractions and temptations of this earth. For this is exactly what the kingdom of Heaven looks like, so if you are not content with only God, then you would not be content in Heaven, and as such you do not deserve the incredible gift that is Heaven, and you will be denied access.