Oecumenius

He says, Nevertheless, you have a few names in Sardis who have not defiled their garments; and they shall walk with me in white, for they are worthy, whom I say I am now postponing and patiently enduring for you. The garments not defiled refer to the bodies of the saints according to what was said by the patriarch Jacob; he washes his robe in wine (Gen. 49:11); and Isaiah says the same: Why are your garments red, as if they were stained in a winepress? (Isa. 63:2-3) The white garment thus symbolizes the purity of the body.

Commentary on Revelation ↗

Bede

But you have a few names in Sardis who have not defiled their garments. He did not say a few people, but a few names. For he calls his own sheep by name; he knows Moses by name, and he writes the names of his saints in heaven.

Commentary on Revelation ↗

Alcuin of York

But I have a few names in Sardis, who have not defiled their garments: and they shall walk with me in white, because they are worthy. Those who do not stain their garments with filth are those who commit no deadly sin after the sacrament of baptism; but since they are rare, it is first said a few names. Where then, Lord, shall they walk with you to? Where from, and where to, if not from you into you? For they walk with you every day, because they follow and observe the footprints of your Passion. …

Commentary On Revelation ↗