Matthew 10:26

“Do not be afraid of them therefore, for there is nothing covered, that will not be revealed, and hid, that will not be known.”

Thus in the light of eternal realities they are not to be afraid of them. They persecute Him, they will therefore persecute them. But in the end all truth and falsehood will be revealed. Then they will be glad indeed that they were on the side of truth, whatever the cost. In the Day when everything will be laid bare, their truth and honesty will be manifested, and their actions will be justified while those of their opponents will be seen for what they were, and will be condemned. For all things are open to the eyes of Him with Whom we have to do (Hebrews 4:13).

Alternately, with the following verse in mind, He may be indicating that whatever men do to them they will not be able to prevent the truth going out. What is at present covered up will have the cover taken off, what is at present hidden will be made known (compare 2 Corinthian 3-4).

‘Do not be afraid of them.' Note the threefold ‘do not be afraid', here and in Matthew 10:28; Matthew 10:31. Because they are His and His Father's they need fear no man, neither their accusations, their calumniations nor their threats, because He is watching over them. The only One therefore Whom they must reverently fear is God, their Heavenly Father, and His Father.