The Church at Sardis had a name that they were alive, and dead. How can we be alive and dead at the same time?
 But she that liveth in pleasure is dead while she liveth.
The Church at Sardis was living in pleasures. In the context of the Spirit's rebuke He warns them their works were flawed and not worthy to be offered to God as anything that would please Him. Our Lord implies that their works were dead works. He warns them to remember how they received and heard. He commanded them to repent.
 And unto the angel of the church in Sardis write; These things saith he that hath the seven Spirits of God, and the seven stars; I know thy works, that thou hast a name that thou livest, and art dead.
 Be watchful, and strengthen the things which remain, that are ready to die: for I have not found thy works perfect before God.
 Remember therefore how thou hast received and heard, and hold fast, and repent. If therefore thou shalt not watch, I will come on thee as a thief, and thou shalt not know what hour I will come upon thee.
 Thou hast a few names even in Sardis which have not defiled their garments; and they shall walk with me in white: for they are worthy.
 He that overcometh, the same shall be clothed in white raiment; and I will not blot out his name out of the book of life, but I will confess his name before my Father, and before his angels.
 He that hath an ear, let him hear what the Spirit saith unto the churches.
Here we get our understanding from what Paul had to say to us about the process one goes through to become a Christian.
 So then faith cometh by hearing, and hearing by the word of God.
We tend to think that once we’ve heard The Word of God and gained faith to believe in it, we’re saved and that’s that, but it isn't. We must continue to grow in faith, hope, and love.
 We are bound to thank God always for you, brethren, as it is meet, because that your faith grows exceedingly, and the charity (love) of every one of you all toward each other abounds;
We must remember that with a walk of faith we grow in experience, and it is experience that the Word of God is true that gives us boldness to move forward in faith to another experience of spiritual growth.
 Therefore being justified by faith, we have peace with God through our Lord Jesus Christ:
 By whom also we have access by faith into this grace wherein we stand, and rejoice in hope of the glory of God.
 And not only so, but we glory in tribulations also: knowing that tribulation worketh patience;
 And patience, experience; and experience, hope:
 And hope maketh not ashamed; because the love of God is shed abroad in our hearts by the Holy Ghost which is given unto us.
Please note that in Romans 5: 1-5 above it is through experience that the essential elements of faith, hope, and love grow. Those of Sardis had become complacent where their faith was concerned…Yes they had believed in God and they had attended church each Sunday, and they had prayed some, and they had given some, but not in faith, and in reality all they were doing was just muddling through their Christian lives giving little more than lip service to God. Their hearts were loveless. Their faith was dead, and their hope was in the world. Those of Sardis had lapsed into a state of mental assent waiting to burst through the gates of Heaven and boast of their achievements through their flesh in the world. The Spirit let them know that He wasn't at all pleased with their lives and many were in danger of having their names blotted out of The Lamb's Book of Life.
When we become complacent we forget the Kingdom of Heaven is vastly different than the world we live in. Those of Sardis had lost their sense of what was spiritual and immersed themselves in that which was entirely of the world. They weren't baby Christians and for that reason were without an excuse before the Spirit.
 Then all those virgins arose, and trimmed their lamps.
 And the foolish said unto the wise, Give us of your oil; for our lamps are gone out.
 But the wise answered, saying, Not so; lest there be not enough for us and you: but go ye rather to them that sell, and buy for yourselves.
 And while they went to buy, the bridegroom came; and they that were ready went in with him to the marriage: and the door was shut.
 Afterward came also the other virgins, saying, Lord, Lord, open to us.
 But he answered and said, Verily I say unto you, I know you not.
 Watch therefore, for ye know neither the day nor the hour wherein the Son of man cometh.
Speaking of the parable above...our Lord knew them not because they had defiled their garments...Rev.3:4... (We defile ourselves when we walk in the world after the flesh until the light of Christ within us departs). Notwithstanding, while in the flesh and of the world, we can regain our Lord’s light of life through repentance. Those of Sardis had ceased to have a spiritual life, a life of faith, a life of hope, (looking forward to our Lord’s coming), and a life of love. They persisted in the foolishness of a flesh walk in the world and were in danger of suffering the same consequences as the foolish virgins above. The Spirit had blotted the names of the foolish virgins out of The Lamb's Book of Life.
Those out of fellowship with our Lord at the Church in Sardis were on the verge of becoming foolish virgins. Please notice the parallel between the Parable of the 10 virgins and the Church of Sardis…even in Sardis there were those with clean garments. They represented the wise virgins that took oil in their lamps. Have you filled your lamp with the Spirit of God?