“The fine linen is the righteous deeds of the saints.”
The book is Revelation, and before us is the marriage supper of the Lamb. The Bride is a certain subset of God’s people (we shall not revisit that discussion in detail here), and others among God’s redeemed are present to celebrate. The Bride has clothed herself with “fine linen, bright and pure”.
It’s the most uplifting picture in several chapters of what is, at times, a very dark book, and it is the great hope of the Church.