Revelation 3:1-3

"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 of being alive, but you are dead. Wake up! Strengthen what remains and is about to die, for I have not found your deeds complete in the sight of my God. Remember, therefore, what you have received and heard; obey it 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."

To the angel of the church in Sardis
Revelation 1:11 ... which said: "Write on a scroll what you see and send it to the seven churches: to Ephesus, Smyrna, Pergamum, Thyatira, Sardis, Philadelphia and Laodicea."

These are the words of him who holds the seven spirits of God
Revelation 1:4 John, To the seven churches in the province of Asia: Grace and peace to you from him who is, and who was, and who is to come, and from the seven spirits before his throne, ...

and the seven stars
Revelation 1:16 In his right hand he held seven stars, and out of his mouth came a sharp double-edged sword. His face was like the sun shining in all its brilliance.

I know your deeds
Revelation 2:2 "I know your deeds, your hard work and your perseverance. I know that you cannot tolerate wicked men, that you have tested those who claim to be apostles but are not, and have found them false." (Ephesus)

you have a reputation for being alive, but you are dead
1 Timothy 5:6 But the widow who lives for pleasure is dead even while she lives.

Remember, therefore, what you have received and heard; obey it and repent
Revelation 2:5 "Remember the height from which you have fallen! Repent and do the things you did at first. If you do not repent, I will come to you and remove your lampstand from its place." (Ephesus)

But if you do not wake up, I will come like a thief, and you will not know at what time
Luke 12:39 "But understand this: If the owner of the house had known at what hour the thief was coming, he would not have let his house be broken into."

(parentheses are mine)

NOTES:
come like a thief - not a reference to the second coming of Christ in this verse.
other references to the second coming: Rev. 16:15; Matt. 24:42-44; 1 Thess. 5:2; 2 Peter 3:10

Scriptures taken from the Holy Bible, New International Version® NIV®, Copyright© 1973, 1978, 1984 by International Bible Society. Used by permission. All rights reserved worldwide.