Revelation 3:3

Remember therefore how thou hast received and heard, and hold fast, and repent. If therefore thou shalt not watch, I will come on thee as a thief, and thou shalt not know what hour I will come upon thee.

How thou hast received - (Colossians 2:6; 1 Thessalonians 4:1; 1 Timothy 6:20.) Sardis is to "remember," not how joyfully she had received the Gospel, but how the precious deposit was committed to her originally, so that she could not say she had not "received" it. Not aorist (as Revelation 2:4, Ephesus, "Thou didst leave thy first love"), but "thou hast received" (perfect), and still hast, the deposit of doctrine. 'Keep' [ teerei (G5083)], "hold fast." Observe the commandment thou hast received.

Heard - (aorist), 'didst hear,' namely, when the Gospel was committed to thee. Trench explains "how," with what demonstration of the Spirit from Christ's ambassadors the truth came to you, and how heartily you at first received it. Bengel, 'Regard to her former character (how it once stood) ought to guard Sardis against the future hour, whatsoever it shall be, proving fatal to her.' But thus the same exhortation would be addressed to Sardis as to Ephesus.

If therefore - seeing thou art so warned; if, nevertheless, etc.

Come on thee as a thief - in judgment on thee as a church; as stealthily and unexpectedly shall be my second coming, as the thief gives no notice of his approach. Christ, in language which in its full sense describes His second coming, describes His coming with judgments on churches and states (as Jerusalem, Matthew 24:1); these judgments being anticipatory earnest of that great last coming. 'The last day is hidden from us, that every day maybe observed by us' (Augustine). Twice Christ spake the same words (Matthew 24:42-40; Luke 12:39); which so deeply sank in the mind of the apostles, that they often repeat them (1 Thessalonians 5:2; 1 Thessalonians 5:4; 1 Thessalonians 5:6; 2 Peter 3:10; Revelation 16:15). The Greek proverb, 'the feet of the avenging deities are shod with wool,' expresses the noiseless approach and nearness of divine judgments, when they are supposed far off (Trench).