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?

Dead flies cause the ointment of the apothecary to send forth a stinking savour. Ecclesiastes 10:1

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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