In Matthew 10, Jesus is sending out his disciples. They are to go to the lost sheep of the house of Israel, and they will experience persecution along the way, from their families, from the synagogues of the cities, and even from Gentile rulers. But Jesus says that such is to be expected, for the world is only treating them as it has treated him: with hatred.
Then, v 26 says something puzzling: "So have no fear of them; for nothing is covered up that will not be uncovered, and nothing secret that will not become known" (NRSV).
How does v 26 relate to the topic of the chapter, which is the mission and persecution of Jesus' disciples? The uncovering of the concealed is obviously relevant to its context, since Jesus offers that as a reason for the disciples not to fear their persecutors.
I checked my E-Sword commentaries, and they had some interesting insights. Essentially, they said that God will publicly vindicate the good character and message of the disciples, even as he reveals the enemies of the Gospel to be truly wicked.
That makes sense, for Jesus says in vv 24-25, "A disciple is not above the teacher, nor a slave above the master; it is enough for the disciple to be like the teacher, and the slave like the master. If they have called the master of the house Beelzebul, how much more will they malign those of his household!" The Jewish communities of first century Palestine viewed the disciples as the scum of the earth. They saw Jesus' followers as bad people, while they considered themselves to be servants of God whenever they persecuted them (John 16:2).
But the disciples were the ones who were doing God's will. And the religious establishment had corrupt motivations that they did not lay bare to the public. They were the corrupt tenants who said among themselves, "This is the heir; come, let us kill him and get his inheritance" (Matthew 21:38; Mark 12:7; Luke 20:14). They wanted to kill Jesus to preserve their own power, and they plotted to do so secretly, since Jesus had popular support (Matthew 26:4; Luke 22:2).
And so Jesus might be saying, "Take courage when people persecute you. Eventually, God will make plain that you are the righteous ones, while he will expose your persecutors as the wicked people they truly are."
Did this happen? On some level, yes. God demonstrated that Jesus was who he said he was when he raised him from the dead (Acts 2:32-36). The rulers of the world showed their corrupt natures when they crucified Jesus (Colossians 2:15). And God indicated his displeasure at the Jewish religious establishment when Jerusalem fell in 70 C.E. (Matthew 21:33ff.). God publicly revealed his favor upon Jesus and his righteous anger at his detractors. God exposed Jesus' followers as righteous, and the enemies of the Gospel as wicked.
But Jesus in Matthew 10:26 may be referring to the last judgment, when God "will bring to light the things now hidden in darkness and will disclose the purposes of the heart" (I Corinthians 4:5). Jesus could be saying that God will vindicate Jesus' disciples and expose the wickedness of their persecutors after the resurrection. After all, Matthew 10 says a lot about post-mortem reward and punishment (vv 15, 28, 39, 41-42).
Or Jesus may be assuring his disciples that the Gospel message will go out, regardless of opposition. Jesus says in Matthew 10:27, "What I say to you in the dark, tell in the light; and what you hear whispered, proclaim from the housetops." "The message will go forth, regardless of what people may do to you," Jesus may mean. "So keep preaching what I teach you!"
The statement that appears in Matthew 10:26 is used in a variety of ways, in both the synoptic Gospels (Mark 4:22; Luke 8:17; 12:2-3) and also the Gospel of Thomas (5, 6, 33). In Matthew 10, it comforts the disciples who will face persecution.