More's principles and faith in God were critically important to him. If he made a promise, an oath, then it was binding - it wasn't just something he said to get him through that moment in his life. According to the author of the play, Robert Bolt, "More was a 'hero of selfhood' because he 'knew where he began and left off, what areas of himself he could yield to the encroachments of his enemies, and what to the encroachments of those he loved' (Bolt's Preface to the play, p. xi)" (cited from the eNotes link below).
Definitely check out the link I cited as it gives very important information regarding the themes of this great play. Good luck!