Revelation 7

John observes an angel preserving a large number of the children of Israel from the impending disaster.  12,000 from each tribe are given God's seal of protection (no small feat, since some of these tribes had their family records destroyed in the Assyrian invasion in the 700s BC).  John then sees a larger crowd, one that could not be counted for its size declaring the great salvation God has wrought and the glory He deserves for providing so great a salvation through His Son, the Lamb.  This crowd is identified as 'ones who come out of the great tribulation and washed their robes in the blood of the Lamb.'  God provides protection, but it is always by the same method - through the atoning sacrifice of His Son.