A Man for All Seasons is the story of the integrity of Thomas More, who stood up for his beliefs against a King, at the cost of his life. This theme can be broken down into sub-themes, including individualism, ethics, and law, which are described in detail at the link below.
King Henry VIII wants More to support his efforts to secure an annulment of his marriage to Catherine of Aragon. More believes that this would be a defilement of the Holy Sacrament of Marriage. Standing firm by his individual beliefs, More refuses to submit to the King's demands.
A staunch Catholic, More derives his ethics from the doctrine that God has supremacy over earthly powers, and that the Pope is His emissary on earth. When the King marries Anne Boleyn even though the Pope has denied his request for an annulment of his previous marriage, More, who holds the high government position of Lord Chancellor, follows his ethical conscience and will not give public approval to the King's actions.
Thomas More is, among other things, a brilliant lawyer, and the essence of his confrontation with Henry is whether papal law or English law takes precedence. More's insistence on recognizing papal law, which represents God's law, as the highest authority leads to his beheading at the hands of the governement.