Sardis had a population of 120,000 and Christ declares to this particular church that "they are dead." In this letter, God shows us his patience and his mercy. He does not requite us according to what we deserve but according to his great mercy.
Philadelphia, on the other hand, receives no rebuke but praise. He then reveals to them that an hour of testing is coming on the whole world and that he will keep them from it, that is, he will give them the strength to endure and be victorious.