And some of them of understanding shall fall, to try them, and to purge, and to make them white, even to the time of the end: because it is yet for a time appointed.
And some of them of understanding shall fall, to try them - the design of affliction. Image from metals tried with fire. So Hezekiah: "In the business of the ambassadors of the princes of Babylon, who sent unto him to inquire of the wonder that was done in the land, God left him to try him, that he might know all that was in his heart."
And to purge. Even in the elect there are dregs which need to be purged out (1 Peter 1:7). Hence, they are allowed to fall for a time, not finally (2 Chronicles 32:31; Luke 22:31). Image from wheat purged or cleared of its chaff by the wind.
And to make them white - image from cloth. (Revelation 7:9, "clothed with white robes ... made white in the blood of the Lamb").
To the time of the end. God will not suffer His people to be persecuted without limitation, (1 Corinthians 10:13, "God is faithful, who will not suffer you to be tempted above that ye are able," etc.) The godly are to wait patiently for "the end" of "the time" of trial; "for it is (to last) for a time yet appointed" by God.