By the end of 1640, King Charles I had become very unpopular. Some members of Parliament forced him to make changes in the Constitution which gave them a bigger say in how the country was governed. From then on, Parliament was split into two factions - Royalists (Cavaliers) who supported the King and Parliamentarians (Roundheads) who wanted political and religious reform.
On 3 May 1641, every Member of the House of Commons was ordered to make a declaration of loyalty to the king and to Parliament. This was ratified next day by the House of Lords. They called it their Protestation against " an arbitrarie and tyrannical government" and another order was made that every Rector, Churchwarden and Overseer of the Poor had to appear in person before the JPs in their Hundred to make this Protestation Oath in person. It was to be a declaration of their belief in "the Protestant religion, allegiance to the King and support for the rights and privileges of Parliament".
They then had to go back home to their own parish where any two of them were to require the same oath of allegiance from all males over the age of 18. The names of all who refused to make the oath were to be noted and assumed to be Catholics.
We have, in the Devon Protestation Returns, a set of amazing documents - something akin to a census even though no women or children are named. An indexed transcription is available in the West Country Studies Library in Exeter.