The church in Sardis needed to wake up and pay attention. If they did not, they would face sudden judgment. The city of Sardis had experienced this kind of surprise attack twice. The first happened when the forces of Cyrus entered the city through a secret tunnel around 549 or 546 BC. The second happened in 189 BC when Antiochus III attacked the city in the same way.
In both cases, the watchmen were not alert, and the city fell. The same would happen to the Christians there if they did not meet God's requirements (see Revelation 20:12; Ephesians 2:10; James 2:20).
like a thief: Like the invaders in the city’s history, Christ would come suddenly (see Revelation 16:15; Matthew 24:43; 1 Thessalonians 5:2; 2 Peter 3:10).