These believers needed to awake from their spiritual slumber, to examine their condition, and to realize their needs (cf. Matthew 24:42; Matthew 25:13; Matthew 26:41). Their city had fallen into enemy hands more than once due to the carelessness of sentries who had relied too much on the town"s natural fortifications. They also needed to strengthen the areas of weakness in their church, which was almost dead. [source][source][source]
As the Ephesians , they needed to remember the rich spiritual heritage of their church and to return to the attitudes and activities their teachers had taught them. Failure to heed these warnings would result in Jesus Christ sending discipline on the believers that would surprise them. It would be similar to the surprise that earth-dwellers will experience at the Second Coming (cf. Matthew 24:43; Luke 12:39; 1 Thessalonians 5:2; 2 Peter 3:10; Revelation 16:15). These Christians were not watching expectantly for the Lord to return. Note the many allusions to Jesus" teachings in the Gospels in this Sardis letter. [source][source][source]