The 1641 Oath of Protestation was when Parliament decreed that everyman, over 18, in the country should swear their allegiance to the Church of England.
Anyone not swearing allegiance would be banned from any public office.
In general, with few exceptions, only males swore. The Oath:

I, …………….do, in the presence of Almighty God, promise, vow, and protest to maintain, and defend as farr as lawfully I maye, with my Life, Power and Estate, the true Reformed Protestant religion, expressed in the Doctrine of the Church of England, against all Popery and Popish Innovations, within this Realme, contrary to the same Doctrine, and according to the duty of my Allegiance, His Majesties Royal Person, Honour and Estate, as alsoe the Power and Privileges of Parliament, the lawful Rights and Liberties of the Subjects, and any person that maketh this Protestation, in whatsoever he shall do in the lawful Pursuance of the same: and to my power, and as farr as lawfully I may, I will appose and by all good Ways and Means endeavour to bring to condign Punishment all such as shall, either by Force, Practice, Councels, Plots, Conspiracies, or otherwise, doe any thing to the contrary of any thing in this present Protestation contained: and further, that I shall, in all just and honourable ways, endeavour to preserve the Union and Peace betwixt the Three Kingdoms of England, Scotland and Ireland: and neither for Hope, Feare, nor other Respect, shell relinquish this Promise, Vow and Protestation.

This event was recorded at parish level and the records which still mostly survive are kept in the House of Lords today.
Transcriptions of this event are available in book form for many, if not all, counties.
An interesting estimate of the population can be made from these lists, in-so-far that if the total number of signees (all male) is multiplied by 4, that is the population.

(The Protestation Returns of 1641–1642 are lists of English males over the age of 18 who took, or did not take, an oath of allegiance “to live and die for the true Protestant religion, the liberties and rights of subjects and the privilege of Parliaments.” These lists were usually compiled by parish, or township, within hundred, or wapentake. They are of importance to local historians for estimating populations, to genealogists trying to find an ancestor immediately before the English Civil War and for scholars interested in surname distributions.)

Edward Rowell, Will: Rowell – Listed in Bythorne D U 6 February 15^th 1641

Thomas Rowell, Keystone, D U. 17.February 18^th 1641

John Rowell, Kimbolton, D. U. 18 February 13^th 1641