They used to mock Islam and its people. They were afraid that Allah would expose them through revelation about them. Until some of them said: "By Allah, we see nothing but the worst of Allah's creation. I wish I had come forward and received a hundred lashes, rather than anything being revealed about us that would expose us." The pronoun in "upon them" and "inform them" refers to the believers. And "in their hearts" refers to the hypocrites. This is correct because the meaning leads to it. It is also possible that the pronouns refer to the hypocrites, because if the surah was revealed regarding them, it is revealed upon them. The meaning of "inform them of what is in their hearts" is as if it is saying to them: "In your hearts is such and such," meaning that it discloses their secrets to them until they hear them widely spread, as if it is informing them of it. It was said that the meaning of "they are afraid" is a command to be cautious, meaning that the hypocrites should be cautious. If you say: "The caution is regarding the revelation of the surah in His saying: 'The hypocrites are afraid that a surah will be revealed upon them,' what is the meaning of His saying: 'Exposing what you fear'?" I say: Its meaning is that it is a manifestation of the revelation of the surah. Or that Allah is exposing what you were afraid of, meaning what you fear to be revealed about your hypocrisy.