William Burkitt on Revelation 3:4

Note here, That though the greater part in Sardis were dead or dying, that is, declining or decaying, yet there were some that kept their innocency, and preserved themselves from error and false worship, from erroneous principles and debauched practices.

Note farther, That these few which keep themselves pure above the rest, are not commanded to separate from the rest. Doctrines crying up purity to the prejudice of unity reject, for the gospel calls for unity as well as for purity.

Note also, The reward promised to such as kept themselves in Sardis pure both from error and vice: They shall walk with Christ in white, like persons of dignity and honour, like kings and conquerors, who of old wore white garments; they shall have the reward of their innocency and uprightness, for they are worthy of it, according to the law of grace, which promiseth it to them: they have walked worthily, with a worthiness of meetness, not with a worthiness of merit; they are therefore meet receivers, though not meritorious purchasers, of this reward.