"Every natural-born subject owes allegiance from birth"Dear Mr P,
The legal concept of allegiance is that every natural-born subject owes allegiance from birth and that allegiance cannot be cast off. The parliamentary oath (or affirmation) gives expression to this allegiance and demonstrates the individual's acceptance of it
It is undoubtedly the case that there have been Members of Parliament who hold republican beliefs but have nevertheless found it possible to take the oath.
The legal requirement is to take the oath in the prescribed form. The criterion is an objective one: the Speaker cannot see into a Member's thought processes, though naturally he would expect the process to be taken seriously and with sincerity.
If a Member claimed to have taken the oath "falsely," the possibility of a contempt of Parliament would arise: that would be a matter for the House.