"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. 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. He who overcomes will, like them, be dressed in holy white garments. I will never blot out his name from the book of life, but will acknowledge his name before my Father and his angels. He who has an ear, let him hear what the Spirit says to the churches.'" (Revelation 3.4-6)
Twice in its history, Sardis had fallen to enemies even despite its vertical walls and fortifications because it had not kept watch. "Remember your past, your commitments and your faith," said the Lord.
They were instructed to, "repent to turn from their sin and come back to God." Failure meant that Christ would come to them when they least expect it "like a thief." Rather than soiled clothing in this cloth-making city, they would be given holy white garments to wear.