Poole on Revelation 7:13, 14: The White Robes of the Elect
Verse 13: And one of the elders answered, saying unto me, What are these which are arrayed in (Rev. 7:9) white robes? and whence came they?
[And he answered] That is, he said (Menochius, Ribera, Grotius, similarly Beza, Drusius), as in Matthew 11:25 and elsewhere (Grotius), after the manner of the Hebrews (Ribera, thus Grotius).
[One (in the place of all, as in Revelation 5:5 [Cotterius]) of the elders] He was [either] a certain Presbyter renowned for the Prophetic gift, by which even the Apostle does not scruple to be taught, as Paul by Agabus (Grotius); or, one of the Bishops (Hammond); or, one of the members of the Church, so that the mutual help which the members supply to each other is signified (Durham). Others: a teacher in the Church: now John in this place, as a disciple, bears the person of the entire Church (Cluverus).
[These, who are they, and whence, etc.] He asks, not as one ignorant (Pareus, thus Gravius), nor as uncertain (Durham), nor so that he might be taught that by John (Cotterius); but as one stirring John to consider carefully the matter (Pareus, thus Durham), and to desire to know (Cluverus, Forbes), as a matter unusual and stupendous (Forbes), most worth of investigation, and useful (Durham); [both] for his and our consolation (Pareus), [and] for the explication of the rest of the Prophecy (Durham): either, so that he might furnish for him an opportunity for considering more accurately the state of the elect (Gravius); or, as one about to test whether John understands it (Cotterius); or, so that John might acknowledge his own ignorance (Forbes, similarly Cluverus), and thus be brought unto docility (Cluverus); or, as one seizing an occasion to teach him thoroughly (Pareus).
What are these, etc.: Not that he did not know, but to try whether John knew, or rather to set John upon inquiring.
Verse 14: And I said unto him, Sir, thou knowest. And he said to me, (Rev. 6:9; 17:6) These are they which came out of great tribulation, and have (Is. 1:18; Heb. 9:14; 1 John 1:7; Rev. 1:5; see Zech. 3:3-5) washed their robes, and made them white in the blood of the Lamb.
[My lord] His address is winsome, as in John 20:15. Thus the Hebrews address even a stranger as אֲדֺנַי/adonai/lord (Grotius). He saw that this one was to be venerated on account of his age, throne, crown, and clothing (Cluverus).
[Thou knowest] That is to say, I do not know, and I wish to learn it from thee (Durham, similarly Piscator, Forbes). This is an example of modesty, and of humility, and also of human ignorance in heavenly mysteries (Cluverus).
[These are (he declares both who and whence they are, in accordance with the double interrogation [Forbes]) they which came (that is, escaped victorious by Divine power [Pareus]: He asked, whence they came; he responds, they are οἱ ἐρχόμενοι, those coming: for they had not yet come out of affliction, for future things were being shown to John: therefore, they were going to come, and at the present time are coming, etc.: However, they had come with respect to the Vision, for these things were represented to John as past [Cluverus]) from the great tribulation] Out of those places, where Stephen, where James of Zebedee, were killed, where they lived in the greatest dangers, vexed in every way. See 1 Thessalonians 2:14; Hebrews 10:32, 33 (Grotius). Out of the spiritual and most grievous servitude of Antichrist (Forbes). From the tyranny of the Beast troubling the saints (Mede’s Works 525, 693). From the persecutions under Antichrist (Durham, similarly Piscator), which the Trumpets signify (Durham). He understands, either, 1. the martyrs alone (Lyra in Cluverus, Gregory and Arethas and others in Ribera): inasmuch as they washed themselves in the blood of the Lamb (certain interpreters in Ribera). But it is to be noted that they are said to have been washed, not by their own, but by the blood of the Lamb (Cluverus). Or, 2. all the elect (Ribera out of Ambrose and Bede and Tichonius and others, Cluverus); as show, both, 1. συνέχεια, the coherence, of the text (Cluverus): and, 2. that all these are washed in the blood of the Lamb (Tichonius in Ribera): and, 3. that it was treated of the souls of the Martyrs already above and separately, Revelation 6:9-11, neither is it necessary to repeat the same thing twice in the same Vision. The afflictions of the pious on this side of death from sicknesses, temptations, mourning, etc., are certainly great (Cluverus).
[And, etc., καὶ ἔπλυναν—καὶ ἐλεύκαναν, etc.] In the place of πλύναντες ἐλεύκαναι, washing to make white. And λευκαίνειν here is not to make white, but to render ornate. Just as wool is made more beautiful by the blood of purple, so also their garments by the blood of Christ. Now, these are called the robes of them, that is, their consciences, tinged by the blood of Christ; not that they endured death, but rather things exceedingly close to death, and thus they imitated Christ (Grotius). And they washed (that is, rendered pure, sanctified [Menochius]) their robes (that is, their vestments and bodies, that is, themselves and their consciences [Menochius out of Lapide]: they procured and renewed holiness [Ribera]: The white robes signify righteousness and purity [Pareus]: By this metaphor he understands the washing of the soul, the garment of which, as it were, is the conscience, either filthy, or pure [Lapide])…in the blood (that is, through the merits [Lapide, Menochius]) of the Lamb. They are not washed (Durham), cleansed (Cluverus), justified (Forbes), by their own innocence (Durham), their own righteousness (Forbes), their works (Durham), or tribulations (Cluverus, Durham), or human merits, like the followers of Antichrist; or by carnal washings and rites, as the Jews were dreaming (Forbes); but only by the blood of the Lamb (Cluverus, Forbes). This washing is of grace, not of nature; of spirit, not of art (Pareus). Repentance and good works through the blood of Christ have the power to cleanse (Ribera). To these that saying of Achmet, Oneirocriticon 231, from the teaching of the Indians, will add light, If anyone, with filth washed away, should appear to himself to have put on garments clean and white, he shall be freed from all anxiety, sickness, affliction, and sorrow (Mede’s Works 1112).
Sir, thou knowest, etc.: John confessing his own ignorance, applies himself to this elder for instruction, who tells him: These were the souls of them that came out of great sufferings and persecution; but he addeth, that they were such as were washed in the blood of Christ. Suffering will not bring us to heaven without having our souls washed with the blood of Christ.
 Greek: Καὶ ἀπεκρίθη εἷς ἐκ τῶν πρεσβυτέρων, λέγων μοι, Οὗτοι οἱ περιβεβλημένοι τὰς στολὰς τὰς λευκάς, τίνες εἰσί, καὶ πόθεν ἦλθον;  Acts 21:10, 11.  Greek: καὶ εἴρηκα αὐτῷ, Κύριε, σὺ οἶδας. καὶ εἶπέ μοι, Οὗτοί εἰσιν οἱ ἐρχόμενοι ἐκ τῆς θλίψεως τῆς μεγάλης, καὶ ἔπλυναν τὰς στολὰς αὐτῶν, καὶ ἐλεύκαναν αὐτὰς ἐν τῷ αἵματι τοῦ ἀρνίου.  Greek: Κύριε.  Acts 7.  Acts 12:2. Purpura is a type of shellfish which yields a purple dye.