Ooooh, something that I know something about. Yay. Ok, a quick proviso, I have not read MacCulloch's book so I don't know exactly what his argument is regarding the English civil war, but I can approximate some of the key historiographical debates about the Civil War

Since you asked about religion, lets start with that. It isn't hugely controversial that religion was a key reason for the outbreak of war in all three kingdoms by 1641 -- the debate is to what extent it assumes primacy as a reason. Now, the three kingdoms of Scotland, England and Ireland were all religiously divided (we will get to the interaction between them later) but the religious debate in England is probably what he is referencing. Broadly, there were three distinct religious groups in England at the time. Catholics, by far the smallest and concentrated in gentry households and the north. Those who supported the Church of England and 'Puritans'. I put Puritans in quotation marks because pinning down what they actually were was almost impossible -- they are the true 'protean beasts' of early modern England. To a certain extent there were doctrinal differences between the Church of Englanders and the Puritans. the former were creedal Calvinists (they argued from Calvin himself that it was impossible to know whether a person was reprobate or saved and therefore that the church could contain both sinners and saved. They were also generally in favour of Bishops and the maintenance of the church hierarchy). 'Puritans' meanwhile tended towards experimental calvinism (arguing from Calvin's disciple Beza that it was possible to know that one was saved) and tended to be Presbyterians. Further, both sides of the debate were shit scared that the other was trying to do them harm. In particular the Puritans feared that people like Laud would try to reintroduce Catholicism (Laud was generally thought to be an Arminian -- a protestant who believed in free will) and wanted to introduce a more ceremonial (Catholic) form of worship with altars in churches, new prayer books and so on.

However, I believe that a lot of this is really quite overblown. Personally I am not convinced that Laud was an Arminian, to me he seems firmly in the tradition of other Conservative protestants like Whitgift, Elizabeth's Archbishop of Canterbury. Indeed, his programme seems to me to be very like the attempt made by James I/VI to bring his kingdoms into religious congruence. Further, the degree of theological unity and the requirement to be united against Catholic threats (France) always ensured a relative degree of Protestant solidarity.

It would also be possible to lay the blame firmly at the feet of one person -- Charles I. For context, the English and Scottish monarchies at the time were intensely personal affairs which required the constant attention of the sovereign to keep on the road and a generally gregarious outlook to manage some very large personalities. The first observation, was that he was an ideological ruler in the worst possible way. He had very clear convictions about his own prerogatives and pursued them doggedly and in ways which really rubbed people up the wrong way. He was also awkward, shy and insecure. So for instance to stop courtiers coming to him personally to discuss matters he had the locks on the Palace of Whitehall changed so he would not have to face them in a way which made him uncomfortable. He also had a tendency to view people who disagreed with him as traitors, which made legitimate expression of grievance difficult. Further, his dislike of open debate led to him governing through institutions like the privy council which he could control absolutely. Finally, he had a reputation as a man who would go back on his promises. This goes back to him being ideological, if you are a divine monarch with certain rights, then there is no reason not to go back on concessions because they were unjustly granted in the first place. So he accepted the Petition of Right in 1628 (which asserted the rights of parliament and subject against him) under pressure and once the furore had passed slightly dissolved parliament and did away with it. The upshot was that as the civil war loomed people like Pym did not think that they could trust the king's word, making them more likely to support armed resistance to charles.

MacCulloch probably didn't talk a lot about the constitutional issues which helped to touch of the civil war, and why should he? But to make a long story short, there were real questions and contradictions floating about concerning the powers of the king and those of parliament. Charles's father James, canny political animal that he was, was able to split the difference during his reign. He repeatedly asserted the divine right of kings. For instance, in his speech from the throne in 1610 he commented said “kings are not only god’s lieutenants on earth, and sit on gods throne but even by god himself are called gods” but makes a clear and firm distinction between original kings and kings such as himself that govern civil kingdoms who are bound to “the observation of the fundamental laws of his kingdom”. His ordinary prerogative is expressed through king in parliament, but he had an extraordinary prerogative (to break laws made by parliament) that could only be used in a national emergency and supplemented, but did not contravene, the ordinary prerogative. He was thus absolute and limited at the same time and understood how carefully he had to use his extraordinary prerogative. His son Charles was not so nuanced. So with the forced loan of 1626 charles simply declared that there was an emergency and used his extraordinary prerogative to levy an extra parliamentary tax to create 240k. the other side (Thomas Scott, member for Canterbury) meanwhile argued that such a measure usurped powers that were meant for the good of the commonwealth for the good of the monarch personally

The final thing to say, which i'm not convinced by but whatever, is the so called 'British problem'. The argument from Conrad Russel is, put simply, that the arrangement by which Charles governed three legally separate kingdoms each with a different faith and very different constitutional arrangements was inevitably going to end in tears because dissident groups in each kingdom could appeal to others to support their cause. To take one example, the Scottish invasion of England during the Bishop's war emboldened Puritans in England to challenge Charles further. While this is very clever, in my view it underestimates the very different views about the nature of sovereign power which Charles and Parliament had.

So my answer to your question is simple. It was a combination of factors, not solely religious, and people at the time recognised this as well. So Pym thought that “liberties of parliament, matters of religion…the affairs of state or matters of property” were the main reasons for the civil war and he was probably right.

I believe there is strong evidence to support the argument of the English Civil War being a religious war. Especially when we look at the pre-civil war era we can find some better answers. From the research I have done, it definitely seemed like a religious war from the perspective of rebels and revolutionaries. I don't believe there is one cause for the civil war, as it seemed to be the clash and timing of many factors (religion, politics, parliament and monarch issues, etc.) that ultimately resulted in civil war, but religion definitely played a part, especially from the rebels and revolutionaries perspective. I wrote a small essay from this point of view, that might give some more insight as to the religious pre-civil war tensions between the rebels and revolutionaries. I am not declaring any EXACT cause or reason for the civil war, but merely giving a different look at the religious perspective of the Civil War (I hope this is helpful in some way):

There are several theories about why the civil war in England came about. Conrad Russell accounts for the civil war as a result of “three long-standing instabilities…multiple kingdoms, problems of religious division, and the breakdown of a financial and political system in the face of inflation and the rising cost of war.” John Morrill accounts for local interests as being the cause and not from the problems coming together at the center, or Court. It is difficult to pinpoint or account for the exact cause of civil war, as seen in the difference of opinions on the matter, but a closer look at the religious ideas and actions of rebels and revolutionaries (RR) may provide a better over-hanging cause to civil war. Simply put by Diana Newton, this was a “war of religion”. In opposition to Charles, RR’s cause was that they were “united by their rejection of Laudian ‘innovation’ and by a genuine fear of a ‘popish plot’”. While the RR did not single-handedly cause the civil war, they heavily influenced it through the illegal press printing, the Protestation, and iconoclasm.

Firstly, a clear way the printing press was utilized for the RR influence to the onset of civil war was through the varying documents produced by the Cloppenburg Press. David Como noted this discovery of Johnsons’, who uncovered several English pamphlets having “identical typeface and a distinctive set of printer’ ornaments – [which] he dubbed it the ‘Cloppenburg Press’” , as “contributing to our understanding of the crisis that led to the outbreak of the Civil War”. This can be seen in the fact that there were several RR groups who had illegal pro-propaganda printed from the same press, the Cloppenburg Press, to promote their different, but unified causes against their sovereignty. For example, the Scottish Covenanters had their manifesto, An Information from the States of the Kingdome of Scotland, printed , several Puritan leaders had their anti-Laudian propaganda printed, including Henry Burton’s 400-page attack on Laud’s treatise and William Prynne’s anti-episcopal pamphlet The Antipathie of the English Lordly Prelacie , and other “radical puritan propaganda by less august figures” all of which were agreeably seen as treasonous efforts and acts against the Monarch. The communicative efforts of the Scottish propaganda, however, to promote their cause is extremely important in the pre-civil war onset: Each official declaration published by Cloppenburg Press was carefully designed to highlight the fundamental unity of the Scottish and English causes; to identify the mutual enemies of the two kingdom, that is, the bishops and popish councilors who had led the king astray; and to emphasize the lawful and orderly nature of the Scot’s proceedings, so as to Allay English fears that the Scots were seditious rebels or self-interested marauders.

It is clear here that there was some kind of political communication being done within the illegal printing press or a large scale. It may not have directly caused the start of the civil war, but it did influence the thoughts and opinions of crucial pre-civil war debates of the time regarding RR causes against the Monarchy. Therefore, the use of illegal printing for religious propaganda was an influencer to the onset of the civil war.

Secondly, the Protestation document Parliament served as another influencer to the onset of civil war against the Monarchy. The Protestation was made by Parliament in May 1641. In summary, “it brought discussion of the constitution, politics, religion, and the law from Westminster, to county assemblies, and into every church and parish.” Nearly a year after it was made, it had extended to the eyes of readers all over the nation. It is clear why this happened, as the Protestation addressed the concerns of an entire nation. This shows the power and influence the document had, not only in its’ content, but in the “manner of its application” , as it “outreached from the governing classes to the governed”. What was so important about it? Since it originated in the political crisis of May 1641 , fears that Charles was going to “dissolve parliamentary liberties and subvert the reformed religion” had already been in existence, and this document gave hope to those fears: “[it] was an armory against popery, a defense against Rome, a bulwark against superstition, and a pledge for a godly Church of England.” This document fed quite well into RR cause of fighting against the Laudian innovations. While the document was not necessarily meant to divide people, “nearly everyone was tendered the Protestation found it something they could subscribe.” It was not specific, but “[it’s words laid] at the heart of the crisis that eventuated in civil war.” Therefore, its influence is clear as to affecting the onset of the civil war, as these vary matters developed into the key aspect of the civil war.

Lastly, the iconoclasm that took place within English communities also influenced the religious RR cause and onset to the civil war. As early as July 1640, “altar rails…in Essex were broken down and carried away by a crowd…” and later “…the troops toured the area around Braintree, pulling down the rails in the godly parishes of Finchingfield, Terling, Little Leighs, and above fifty other church” as well as in Suffolk. There is not an exact number of incidents that occurred , but what is known is that they happened, in part due, to the “conflict between aggressive Laudian ministers and combative godly parishioners…” Their influence lies in that public worship ceremonies became a debate on who would enforce the Laudian ceremonialism regime and who would not , which would have been an important issue for the Monarchy to handle in terms of who supported them. Any defiance against Archbishop Laud’s religious administration would clearly be seen as rebellious, so violently taking down altar rails in churches would be a definite sign to the RR cause. Thus, iconoclasm, while more violent than pamphlets and the Protestation, was a clear way the RR influenced the religious discord leading up to civil war. Iconoclasm was a direct rebellion to the Laudian innovations set forth by the Monarchy, which gave strength to the RR cause.

From a religious perspective as to the main cause of the civil war, it would appear the RR had a heavy influence. The pamphlets and books from illegal printing presses and the Protestation seemed to communicate and spread RR causes on a national stage. Whether they brought in more people to their side is not important, it’s the fact that these two methods caused reactions and discussion that was important to building the tension that led to the break of civil war. Iconoclasm was clearly a much more abrupt way to build tension and communicate a message of opposition. Still, iconoclasm did bring about important central debates to the ceremonial practices of religion that were fought for and about during the civil war. In all of the influencers, the opposition against the Monarch’s Laudian religion was clear, and this helped build the RR cause, which in return built the tension that eventually snapped into civil war.

Bibliography

Como, D. “Secret Printing, the Crisis of 1640, and Origins of Civil War Radicalism.” Past & Present 196 (2007): 37-82.

Cressy, D. “The Protestation Protested, 1641 and 1642,” Historical Journal 45, no. 2 (2002): 251-280.

Morrill, J. on “The Coming of War.” in M. Todd. Reformation to Revolution. London and New York: Routledge, 1995.

Newton, Diana. “The Civil War and the Interregnum.” In Papists, Protestants, and Puritans. Cambridge: University of Cambridge, 1998.

Russell, C. on “England in 1637.” in M. Todd. Reformation to Revolution. London and New York: Routledge, 1995.

Spurr, John. Religion, Politics, and Society in Britain: The Post-Reformation 1603-1714. Essex: Pearson Education Limited, 2006.

Walter, John. “Popular Iconoclasm and the Politics of the Parish in Eastern England, 1640- 1642.” Historical Journal 47, no. 2 (2004): 261-290.