We may understand this to refer to justification. "They will walk in white"; that is, they will enjoy a constant sense of their own justification by faith; they will understand that the righteousness of Christ is imputed to them, that they have all been washed and made whiter than the newly-fallen snow.
Again, it refers to joy and gladness, for white robes were holiday dress among the Jews. They who "have not soiled their garments" will have their faces always bright; they will understand what Solomon meant when he said, "Go, eat your bread in joy, and drink your wine with a merry heart, for God has already approved what you do. Let your garments be always white."1
The one who is accepted by God will wear white garments of joy and gladness while they walk in sweet communion with the Lord Jesus. Why are there so many doubts, so much misery and mourning? It is because so many believers spoil their garments with sin and error, and as a result they lose the joy of their salvation and the comfortable fellowship of the Lord Jesus; they do not walk here below in white.
The promise also refers to walking in white before the throne of God. Those who have not soiled their garments here will most certainly walk in white in heaven, where the white-robed crowd sings perpetual hallelujahs to the Most High. They will possess joys inconceivable, happiness beyond a dream, bliss that imagination knows not, blessedness that even the stretch of desire has not reached.
"Those whose way is blameless"2 shall have all this--not of merit, nor of works, but of grace. They shall walk with Christ in white, for He has made them "worthy." In His sweet company they will drink from the fountains of living waters.