Today
The Sardisians were called to remember what they have received and heard. What they 'received' was the apostolic tradition of the gospel; what they ‘heard' were the teachings of the apostles and prophets who brought the gospel to them. The Sardisians were not holding to the word of Christ.
For them repentance was the only way out of certain and final death. So they were to repent by restoring the gospel and the apostolic doctrine to its authority over their lives. This would mean they would once more start obeying (tereo, 'keep' or 'watch') the truth of Christ's word. Today's Church needs to hear this challenge to take the word of Christ seriously. Unless the church at Sardis repents, Christ says that he will come to them in judgment
'as a thief' - ie, by surprise - just as Sardis had been attacked and defeated by Cyrus long before. 'As a thief' should probably not be taken as referring to the Second Coming but to Christ's coming against them (opposing them) in judgment. When Jesus speaks, let’s be quick to respond.