Honesty and secrecy
The idea of honest and secrecy is the main theme of the story, because the main character is defined by his thirst for truth. In the fullness of time, that thirst for truth comes full circle, and he realizes that the person whom he wants to trust most—his own flesh and blood (indeed, the even share their name!)—is perhaps the most scandalous, selfish person of the bunch. He learns that not only was his father's review of his book unwarranted, it was also motivated by a guilty conscience. This comes to Gabriel, Jr, as a crisis of trust. If he can't trust his father, who can he trust?
Moral compromise
The father does finally manage to confess his sins, so to speak, before he dies. But it's interesting that he doesn't go to a priest or monk or something. He finds a woman to fall in love with, and they have an affair, even though they're married to other people, and only in that context is he comfortable admitting his secret—while indulging in a secret relationship. Then she tells everyone, so in the end, his involvement with the government turns out to be a public issue after all, though he did his best to conceal it all his life. This is a thematic picture of moral compromise—knowing the right thing to do, but taking an easy way out, basically. He knows that he'll feel better once he's honest, but he can only manage honesty while he's cheating on his wife, so how honest is he?
The truth value of common history
By the end of the book, the reader should see this theme emerge: Just because we have a generic understanding of what happened in the past, that doesn't necessarily mean that we have high grade expository journalism about the past. We rarely have a diverse interpretation of the past, even though there are many points of view to consider. The value of this theme in the novel is that we should not pretend we understand the past very well at all. Even the reporter felt sure of his understanding, and then in one fell swoop, he learned his father was the villain all along. He was wrong.
