How did those who survived the great tribulation make their robes white?

0 How did those who survived the great tribulation make their robes white?

Behold, the people is one, and they have all one language ... Now nothing will be restrained from them, which they have imagined to do. Genesis 11:6

These are they which came out of great tribulation, and have washed their robes, and made them white in the blood of the Lamb. Revelation 7:14

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