In a previous study I demonstrated that the church at Sardis allowed herself to fall in disrepair, just as the ancient city did, before it was conquered. Her works weren’t perfect before God, and she failed to watch and pray, which placed her in a dangerous position. Jesus told the church to do three things: 1) remember how they had both received and heard; 2) hold fast; and 3) repent (Revelation 3:3). How would attending to these three matters affect the dangerous state in which Christ found her in Revelation 3?
Recalling how they had heard the Gospel and how they had received it would help them to understand how they should live in Christ now (Colossians 2:6). It was necessary that they recall how they first became related to Jesus and, now, build themselves up in that relationship by trusting him to work out his will in their daily lives. They also needed to show they weren’t ashamed of such a relationship (cf. Mark 8:38) by preaching the Gospel even in seasons of persecution, and in all these things to be thankful (Colossians 2:7).
Nothing is said specifically about the church in Sardis that would help us understand what had brought them to the place where they would receive such a rebuke by Christ. However, all the churches are told to read what the Spirit says to the seven churches (Revelation 3:6; cf. 2:7, 11, 17, 29; 3:13, 22). In other words, there would be something of interest to all the churches in each of the letters. With this in mind, we might ask ourselves why the church of Sardis needs to be reestablished in the faith. What had they neglected that they needed to recall to mind? I believe it is error that causes one to forget the truth. If this is logically sound, then the church at Sardis may have been harassed by false teachers, (2Peter 3:17). So, the very same type of Jewish evangelists (Jezebel) who had plagued the church at Thyatira (Revelation 2:20) might also have been seducing believers in Sardis. Therefore, the command to recall how they had received and heard (Revelation 3:3; cf. 2Peter 3:17) would be of a great help to them in recognizing their error.
They were to hold fast what they remembered, namely, how they had received and heard (Revelation 3:3). Remembering that they had once been the enemies of the Lord, alienated from him (Colossians 1:21), would help them to understand that the Law, which was preached by Jezebel (Revelation 2:20), was powerless to restore them to God. The fact is, they were the enemies of faith while they were under the Law. Trusting wholly in Jesus would set them apart as innocent and irreproachable in his sight, established and unmovable in the Gospel (Colossians 1:22-23, 27). In so doing they would abound in good works for the sake of the Gospel (1Corinthians 15:58), and unmovable in him, even in seasons of persecution (1Peter 5:10).
The Messianic believers needed to repent of forgetting what they had learned, as though it had no value. Now they needed to recall, what they had once known and earnestly contend for the faith, which was once preached to them. Men had secretly crept into the church at Sardis. Nevertheless, the believers were warned such a thing would take place, and that such men would deny the only Lord and Savior, Jesus Christ (cf. Jude 1:3-5; Acts 4:12; 2Peter 2:1).
Jesus said that, if those in Sardis didn’t watch, he would come to them as a thief, and they wouldn’t know the hour he would come. In this context, Jesus seems to imply that at the time the book of Revelation was written, the day and the hour of his coming and the end of the age (viz. Matthew 24:3, 36) was known or that it would be known soon. Nevertheless, if Sardis wouldn’t watch, Jesus said he would come to them as a thief. That is, they would be unable to discern the time of his coming, and they would receive the same judgment Jesus’ enemies would receive (cf. Jude 1:5-7), meaning they would suffer the same fate as that evil generation that had rejected him (Matthew 23:32-36; 26:64; Mark 8:38).
 Remember, watching involved seeking to understand the word of God and accepting his reproof (Habakkuk 2:1; Proverbs 8:34), taking that word and telling it to others (Psalm 102:7; Isaiah 21:6; Acts 20:31), even when doing so involved persecution (2Timothy 4:5).