Albert Barnes on Matthew 10:26

Fear them not, etc. He encouraged them by the assurance that God would protect them, and that their truth and innocence should yet be vindicated. It is probable that the declaration, "There is nothing covered," etc., was a proverb among the Jews. By it our Saviour meant, that their innocence, their principles, and their integrity, though then the world might not acknowledge them, in due time would be revealed; or God would vindicate them, and the universe would do them justice. They were, then, to be willing to be unknown, despised, persecuted, for a time, with the assurance that their true characters should yet be understood, and their sufferings appreciated.

(r) "for there is nothing" Mk 4:22, Lk 12:2,3, 1Cor 4:5