Sir Thomas More is forced to appear before a royal commission so he can swear personal allegiance to the parliamentary Act of Succession.
This law declares Princess Mary, daughter of the King by Catherine of Aragon, a bastard, and places the unborn princess Elizabeth on the direct line of succession to the throne.
Accepting the Act of Succession means agreeing that the marriage of King Henry VIII and Catherine of Aragon was never valid while the new marriage with Anne Boleyn is valid. This More cannot do. He accepts that Parliament has the authority to declare Anne Boleyn the queen of England, but that's as far as he'll go.
This provides his enemies with enough ammunition to get Thomas More charged with treason. Legally speaking, the evidence against More is weak at best and the primary witness against him will almost certainly be a liar, but the jury won't dare find him innocent.