God never lumps the faithful in with the errant in his assessment. Even in the midst of a wicked culture, there are those who trust in Him which He will never forsake (Gen. Gen. 6:9; Gen. 18:25; Gen. 19:22; Jos. Jos. 6:17).
not defiled their garments
Before coming to faith, these few were wearing filthy rags in comparison to the righteousness required by God (Isa. Isa. 64:6). Having trusted in Christ, His righteousness has been imputed to them and they are positionally holy. Even then, in their walk, they could become defiled by fleshly activities of the world (Jas. Jas. 1:27; Jude Jude 1:23). By confessing their sins, these few had steadfastly remained in close fellowship with God (1Jn. 1Jn. 1:9).
shall walk with me
The picture of walking is two-fold: (1) the faithful are presently guided by the Spirit to walk in His ways (Gen. Gen. 5:22; 1S. 1S. 2:9; Job Job 23:14; Ps. Ps. 37:5, Ps. 37:23; Ps. 40:2; Ps. 66:9; Ps. 119:133; Pr. Pr. 3:6; Pr. 16:9; Pr. 19:21; Pr. 20:24; Isa. Isa. 2:3; Isa. 30:21; Isa. 48:17; Jer. Jer. 6:16; Jer. 10:23; Rom. Rom. 8:1, Rom. 8:4-6; Gal. Gal. 5:16); (2) in the future, the redeemed will have full fellowship with God as did Adam in the Garden of Eden (Gen. Gen. 3:8; Rev. Rev. 21:3+; Rev. 22:3-4+).
They will wear white garments in contrast to the filthy garments they wore prior to coming to faith:
Now Joshua was clothed with filthy garments, and was standing before the Angel. Then He answered and spoke to those who stood before Him, saying, Take away the filthy garments from him. And to him He said, See, I have removed your iniquity from you, and I will clothe you with rich robes. And I said, Let them put a clean turban on his head. So they put a clean turban on his head, and they put the clothes on him. And the Angel of the Lord stood by. (Zec. Zec. 3:3-5)These are those whose sin has been atoned by the blood of Jesus, who wash their garments in the blood of the Lamb. Those at Sardis in Johns day are to be followed by a constant stream of faithful throughout history culminating in those who stand firm to obtain the crown of life during the Great Tribulation (Rev. Rev. 7:9-17+). Those slain for the word of God and for the testimony they held are given white robes to wear (Rev. Rev. 6:9-11+). The white color speaks of the righteousness of Christ, but also of the marriage garments worn by His bride and those who attend the wedding feast who are found to be spotless and without blemish and white and clean:
But when the king came in to see the guests, he saw a man there who did not have on a wedding garment. (Mtt. Mat. 22:11)
Let us be glad and rejoice and give Him glory, for the marriage of the Lamb has come, and His wife has made herself ready. And to her it was granted to be arrayed in fine linen, clean and bright, for the fine linen is the righteous acts of the saints. . . . And the armies in heaven, clothed in fine linen, white and clean, followed Him on white horses. (Rev. Rev. 19:7-14+)
Husbands, love your wives, just as Christ also loved the church and gave Himself for her, that He might sanctify and cleanse her with the washing of water by the word, that He might present her to Himself a glorious church, not having spot or wrinkle or any such thing, but that she should be holy and without blemish. (Eph Eph. 5:25-27)
they are worthy
Their worthiness is found in the worthiness of Him on Whom they have believed. Those who were at one time invited to the wedding, but rejected the Bridegroom were not worthy to attend. Then he said to his servants, The wedding is ready, but those who were invited were not worthy (Mtt. Mat. 22:8). Those with faith in Christ are adopted into Gods family and are found worthy to attain the resurrection of life (Luke Luke 20:34-36, the first resurrection, see commentary at Revelation 2:11).
Gods Word does not refuse to ascribe a worthiness to men (Mtt. Mat. 10:10-11; Mat. 22:8; Luke Luke 20:35; Luke 21:36; 2Th. 2Th. 1:5, 2Th. 1:11); although this worthiness must ever be contemplated as relative and not absolute; as resting on Gods free acceptance of an obedience which would fain be perfect, even while it actually is most imperfect, and on this his acceptance and allowance of it alone.1
1 Richard Chenevix Trench, Commentary on the Epistles to the Seven Churches in Asia (Eugene, OR: Wipf and Stock Publishers, 1861), 164.