Adam Clarke on Revelation 7:14

Sir, thou knowest - That is, I do not know, but thou canst inform me.

Came out of great tribulation - Persecutions of every kind.

And have washed their robes - Have obtained their pardon and purity, through the blood of the Lamb.

Their white robes cannot mean the righteousness of Christ, for this cannot be washed and made white in his own blood. This white linen is said to be the righteousness of the saints, Rev 19:8, and this is the righteousness in which they stand before the throne; therefore it is not Christ's righteousness, but it is a righteousness wrought in them by the merit of his blood, and the power of his Spirit.