MORE: I can't give in, Howard. Our friendship's more mutable than that.
DUKE: Oh, the one fixed point in the world of turning friendship... is that Thomas More won't give in.
MORE: To me it has to be, for that's myself. Affection goes as deep in me as you, I think. But only God is love right through, Howard, and that's my self.
The Duke thinks More is placing mere stubbornness as a higher virtue than friendship. But from More's perspective, he's being true to the religious principles that are so essential to his own being that they are identifiable with his own self. Those religious principles are a manifestation of God's love, which reaches right into the essence of the human being.
DUKE: Oh, confound all this! I'm not a scholar. I don't know if the marriage was lawful or not... but damn it, Thomas, look at these names. Why can't you do as I did and come with us for fellowship?
MORE: And when we die, and you are sent to heaven for doing your conscience... and I am sent to hell for not doing mine, will you come with me, for fellowship?
Norfolk gets slapped—slapped right in the face with an old-timey 15th-century glove. (Well, metaphorically). More phrases his retort in an ironically amusing way—in the scenario he proposes, Norfolk is going to heaven for following his conscience. But is Norfolk following his conscience or just taking the route of convenience along with everyone else?
DUKE: All right. We are at war with the Pope. The Pope's a prince, isn't he?
MORE: He is. He's also the descendant of St. Peter, our only link with Christ.
DUKE: So you believe. And will you forfeit all you have... which includes the respect of your country, for a belief?
MORE: Because what matters is that I believe it, or rather, no... not that I believe it, but that "I" believe it. I trust I make myself obscure.
More emphasizes the importance of being true to himself. It's not that he's proud about his beliefs—it's that those beliefs represent the deepest inward truth of his own self. He can't compromise them without damaging his inner being in God's eyes.
MORE: […] I will not give in, because I oppose it. Not my pride, not my spleen, nor any other of my appetites, but "I" do, "I." Is there in the midst of all this muscle, no single sinew that serves no appetite of Norfolk's, but is just Norfolk? There is! Give that some exercise, my lord!
MORE: ... because, as you stand, you'll go before your Maker ill-conditioned!
More is accusing his friend of being spineless, more or less. That may be the case—or it may be that Norfolk's religion simply isn't very institutional and he just assumes the new church will be as good as the old one. At any rate, it's unclear to More where Norfolk's "self" really lies and whether it is founded on any unmovable principles. Is Norfolk willing to be absolute about anything?
MARGARET: Father. "God more regards the thoughts of the heart than the words of the mouth." Well, so you've always told me.
MARGARET: Then say the words of the oath, and in your heart think otherwise.
MORE: What is an oath then, but words we say to God? Listen, Meg. When a man takes an oath, he's holding his own self in his own hands like water. And if he opens his fingers then, he needn't hope to find himself again. Some men aren't capable of this, but I'd be loathed to think your father one of them.
Margaret might make a good case—but she's also being a bit of a sophist, twisting good logic to bad ends (as far as More is concerned). She's saying that More's inner self and outer words and actions are two different realms: He can believe one thing and say another. But for More, this division isn't a good thing—he thinks it's his moral duty to make his outward actions consistent with his inner self.
MARGARET: If you elect to suffer for it, you elect to be a hero.
MORE: That's very neat. But look now. If we lived in a state where virtue was profitable, common sense would make us saintly. But since we see that avarice, anger, pride and stupidity, commonly profit far beyond charity, modesty, justice and thought... perhaps we must stand fast a little... even at the risk of being heroes.
Margaret also tries to talk More into signing the oath by claiming he's making himself a hero by not doing it—potentially stoking his vanity. But for More, he's not trying to be a hero—it's merely incidental to the fact that he needs to follow these principles. The point is to be true to himself, and he's not attached to how society views him afterwards, hero or no.
CROMWELL: Now, we plainly see you are malicious!
MORE: Not so. I am the King's true subject... and I pray for him and all the realm. I do none harm. I say none harm. I think none harm. And if this be not enough to keep a man alive... then in good faith, I long not to live. Nevertheless... it is not for the supremacy that you have sought my blood... but because I would not bend to the marriage!
Finally, after he's convicted, More breaks his silence. He explains that he's not threatening the king's right to rule—he just thinks that the king's marriage is wrong (since it goes against the Pope's ruling). But the king has tied the question of the marriage to the question of his own supremacy, making them indistinguishable… and that's what kills More.