In A Man For All Seasons, Sir Thomas More is a a man who stands upon his convictions rather than submit to an oath he believes to be false.  More, a devout Catholic, was also prominent in the court of King Henry VIII.  When Henry wished to divorce his first wife, Catherine of Aragon, and take a second wife, Anne Bolyn, he needed a dispensation from the Catholic Church.  The problem was that the Pope had already granted Henry a dispensation to allow him to marry Catherine in the first place.  You see, Catherine was initially married to Henry VII's elder son, Arthur.  When Arthur died prematurely, after his father had already taken possession of Catherine's dowry, the king (Henry VII) wasn't really interested in sending her (and her dowry) back to Spain.  Hence the dispensation from the Pope!  Now Henry VIII wanted the Pope to dispense with his prior dispensation.  His argument was that the marriage wasn't lawful because of a scriptural reference forbidding a man to take his brother's wife as his own.  (cf. Mark 6:17-18)  (There was also a scriptural case that could be made that he not only could, but should do so.  This is a part of the Levitical code from the Old Testament, and it is questionable whether it should apply in a case like Henry's and Catherine's.)  As the divorce effort proceeded, an effort very unpopular with the people of England who had embraced the noble Catherine as their queen, Henry required that all men swear an oath recognizing Catherine (now termed the Princess Dowager) as properly Arthur's widow and not legally or morally entitled to be either Henry's wife or queen, Anne Bolyn as the new queen, Mary (Catherine and Henry's daughter) as a bastard, and most critically Henry as the supreme head of England and of England's church.  Here was the rub.  Most people could go along with the first three parts of that simply as a matter of political expedience and as a question still open to debate.  Others, like Sir Thomas More, could not...indeed would not.  As good Catholics, they still recognized the authority of the Pope in religious matters.  They also regarded the Pope as being in the direct line of the Apostle Peter.  To turn their backs on the Pope was to turn their backs on the Church.  As the Pope was Christ's representative on earth, to deny him was tantamount to denying Christ Himself.  The Bible speaks more than once of our citizenship being connected to the Kingdom of Heaven more than to an earthly one.  In the passage quoted above (Ephesians 2:19-22), the Apostle Paul is in the midst of writing on the oneness of God's kingdom (no longer separated into Jew and Gentile) in Christ.  He even quotes Jesus who referred to himself as the cornerstone or capstone (cf. Matthew 21:42).  This is a theme common in the teachings of the first century church right down to the present day.  It's a theme picked up by the Apostle Peter in his first general epistle (1 Peter 2:4-11).

In the same letter, Peter goes on to describe Christians as "aliens and strangers in the world" (1 Peter 2:11)  In other words, the Christian's primary citizenship is not of this world but of the Kingdom of Heaven.  This doctrine has put Christians, like Sir Thomas More, at odds with any temporal government that would put itself in the place of Christ.  More's loyalty was to Christ first, rather than to Henry.  This rejection of Henry's headship of England's church is ultimately what cost him his head.