A major book would be "The Reformers and their Stepchildren" by Leonard Verduin. The "Reformers" would be Luther, Zwingli, Melanchthon, Calvin, etc. The "Stepchildren" would be the anabaptists, baptists and all who are today non-conformists, such as Pentecostals, Free church, essentially all who neither have nor seek any special privilege from "the State", they do not desire a "state religion" either for themselves or other religious groups. [I have to say the book upset me, because I had rather thought the Reformers rediscovered the idea of "the freedom of religion", and we owe our religious freedoms and freedom of speech, etc, to them... how wrong I was!]
The argument of the book is that the Reformers carried on in precisely the same steps where the Roman Catholic Church they had just condemned had been walking: rather than condemn Christian sacralism outright, they merely swapped Roman Catholic sacralism with Protestant sacralism. This led to the persecution and deaths of non believers, such as Servetus at the hands of Calvin; the deaths of Christian believers, such as the public drowning of the fine Christian Felix Manz in Zwingli's Zurich [Zwingli said "Let him who talks of going under (i.e. in adult baptism) go under (i.e. be put to death by drowning)" {Verduin, page 217}]; and the general persecution of the anabaptists at the hands of the Reformers and their followers. After the burning of Servetus, Melanchthon, another reformer, wrote to Calvin praising him:
"the Church owes and will always owe a debt of gratitude to you for having put the heretic to death" (Verduin's "The Reformers and their Stepchildren", page 52)
In terms of religion, the most successfully Christian country in the world today in terms of adherents of one sort of Christian denomination or another would be the USA where the constitution specifically forbids any favouritism to any one religious organisation.
The Roman Catholic Church has never repudiated the idea of a Church-State union, with the RC Church having the support of the State, enabling the oppression of those who will not conform. Despite this, many Roman Catholics would agree that the RC Church today is more healthy in the USA than in most other countries in the world.
The "Medieval" order, medievalism, can also be called "Christian sacralism". Verduin also calls it "Constantinianism" derived from the name of the Roman Emperor who is commonly associated with effecting the change in the status of Christianity from an illegal cult worthy of persecution to the state religion of the Roman Empire. This new arrangement between the Emperor and the church leaders of those times thankfully brought an end to the persecution of Christians for the most part, but it immediately introduced the persecution of those who insisted on continuing in the worship of the old Roman gods. (It also started the persecution of those Christians who were not happy with the new arrangement, such as the Donatist Christians of North Africa.)
These are the some of the problems with sacralism: it leads to the persecution of those who won't conform - as Franklin Littell writes in the foreword to Verduin's book "to compel men by law to support and participate in religious exercises in which they have no faith is unworthy of Christians who put their trust in the Sword of the Spirit".
It leads to the loss of free speech and the loss of the freedom of religious expression.
It leads to the moral corruption of the religious organisations approved by the State, because these denominations in process of time gain a power, prestige, security and wealth which is attractive to ungodly men, drawing them in and causing them to seek advancement within the denomination.
In addition, when a person is considered to be a member of "The Church" simply because they are a citizen of the State a difficulty of church discipline arises: there is no longer any means of removing them from the Church except to remove them from the State: in such a situation the only way to discipline is to put them to death. But it must be obvious that if the holiness commanded in the New Testament is to be taken seriously there would be rather a lot of people that would need to be put to death.
In consequence a Christian sacralist society loses the ability to take seriously the Bible's standards on holiness.
(By the way, it could be argued that the modern western society does in fact have a form of medievalism: most western States, to greater and lesser extents, do in fact promote a religious point of view which could be viewed as an attempt to create a "unified" society, and these states in the process use many of the legal instruments of the state to promote the unifying ideology. It is just unfortunate for theists that the new unifying religious point of view is Atheism. For instance, over the last fifty or so years in the USA, where the State is prohibited by the First Amendment from promoting any particular religious point of view or denomination, the Supreme Court, an instrument of the State, has rigorously promoted Atheism in schools, colleges, and society at large.. the consequence of a hugely perverse interpretation of the First Amendment.)
New Testament arguments against the idea of a Christian sacralist society
In the Old Testament the Israelite nation under Moses was a sacralist society: to be an Israelite was to be required to follow Jehovah, and failure could lead to death, even for picking up a few sticks on the Sabbath day (Numbers 15:32-36). When Elijah sought repentance he did not go round preaching seeking the repentance of one Israelite here and one Israelite there.. he sought the repentance of the whole nation, because the whole nation was "the church", and the leader of the nation was the leader of "the church": before there could be any real improvement in "the church" the leader, King Ahab, needed to either repent or to die. Of course, the OT Israelite sacralist state, like all sacralist states, could only enforce an outward, superficial, and largely hypocritical obedience.
But from the New Testament it ought to be clear that "Christian sacralism" is a contradiction in terms: the two concepts are at loggerheads against each other:
First, the term "ecclesia" means "called out", i.e. called out from the society of which the believer in Christ is otherwise a member. It means "called out" to be visibly different from those who have not bowed the knee to Christ; it means called out to be holy, to be set apart for Christ. It is this calling out that identifies Christ's kingdom as "the Kingdom of God". It is not a kingdom that relies on any earthly power or privilege; it does not rely on the use of compulsion or the power of the sword; it is a kingdom which relies on the power of the Spirit; the members of this kingdom began to be part of it by their own choice and continue to be part of it by their own free choice. This kingdom does not seek to impose itself by any force; and so our Lord said to Pontius Pilate "My kingdom is not of this world, else would my servants fight" (John 18:36); and to Peter "put away your sword" (Matthew 26:52); the ways of the Old Testament prophets before the coming of the kingdom of God are not appropriate in the era of the kingdom of God (compare 2 Kings 1:10-12 with Luke 9:54-56).
The New Testament form of church discipline is not possible in a sacralist society. "If he neglect to hear the church, let him be unto thee as an heathen man and a tax-collector" (Matthew 18:17b in 18:15-17). If the whole of society is to be considered as belonging to the church then there isn't any place for the unrepentant to be removed to. Another passage, on the same lines, which clearly neither recognises nor can be practiced in a sacralist society is 1 Corinthians 5:9-13:
I wrote to you in my epistle not to keep company with sexually immoral people. Yet I certainly did not mean with the sexually immoral people of this world, or with the covetous, or extortioners, or idolaters, since then you would need to go out of the world. But now I have written to you not to keep company with anyone named a brother, who is sexually immoral, or covetous, or an idolater, or a reviler, or a drunkard, or an extortioner—not even to eat with such a person. For what have I to do with judging those also who are outside? Do you not judge those who are inside? But those who are outside God judges. Therefore “put away from yourselves the evil person.” (1 Corinthians 5:9-13 NKJV)
In Christian sacralism there is no inside and outside.
The ideas of this "Patrick Deneen", who I have never heard of, remind me of the "Reconstructionist"/"Dominion Theology"/"Theonomy" movement which has its origin I believe in some who hold to (Protestant) postmillenialism such as Drs Greg Bahnsen, Rousas Rushdoony and Gary North. (Francis Schaeffer, it is said, was deeply influenced by the movement.) These hold that Christians should be seeking to reconstruct society on Old Testament lines, with everyone subject to the laws and punishments of Moses........................ouch.
Books arguing against Greg Bahnsen, et al would be:-
"World Dominion - the High Ambition of Reconstructionism" by Dr Peter Masters;
"Dominion Theology, Blessing or Curse?" by H. Wayne House and Thomas Ice;
"Theonomy: a Reformed Critique", edited by William Barker & W. Robert Godfrey.
See also https://en.wikipedia.org/wiki/Sacralism
It seems to me that Christian sacralism has always been promoted with a view to the avoidance of persecution for Christ's sake. As such it will always be appealing till the world's end. But the scriptures say that "all who will live godly in Christ Jesus shall suffer persecution" (2 Timothy 3:12). Persecution, then, goes with the territory of true faith in Christ. If anyone wants to avoid this persecution at all costs then there is no compulsion.. they can deny the Lord and leave him.
Christian sacralism gets promoted on the pretext of seeking a political unity or stability within society. This is a horribly insulting misuse of Christ and the Gospel. Christ's work and Gospel are designed for much greater things: to bring individual sinners into a right relationship with God and the saving of an eternal soul. It was never the aim of the Gospel to bring harmony to society. Our Lord said "Do not think that I came to bring peace on earth. I did not come to bring peace but a sword." (Matthew 10:34) It does not mean his followers will literally carry a sword, but the Gospel stirs up antagonism and opposition from sinners, not peace.
Where Christian sacralism is practiced evangelism is compromised. For who is there to evangelise except those who "The Church" says are Christian already?
Where those in political power seek its practice they also seek as a consequence to deny true Christians the ability and right to be visibly set apart from the rest of society. The Church of those who have truly repented and believed in Christ, and who condemn sin, will always sooner or later be thrown into a fiery furnace and a den of lions by a hostile world that calls itself "The Church".
In the sphere of politics the western world, the "free world", holds that individuals should as far as possible be able to choose who rules them by means of free democratic elections.
Further, in the sphere of commerce and business, the western economic theories of "Free Market economics" hold that individuals should be free to choose who to buy from and who to sell to without coercion; and these theories of capitalism lead to the recognition that competition between businesses provides more choice for consumers leading to a struggle to produce better products and lower prices. It is a commonplace understanding that monopolies should not be allowed to develop whereby consumers have no choice about who to buy a specific product from; so in the UK for instance there exists the Competition and Markets Authority which needs to give legal approval when two large companies want to merge.. the CMA will investigate to see if the proposed merger will be detrimental to consumers with the resulting merged company being so dominant in the market that consumers have little choice but must buy its products nomatter what the price. Such regulations not only provide more choice to consumers, they also in the long term produce companies which are more competitive and more useful to society at large.
The same aim of providing free choice to individuals should exist in the sphere of religion, religious organisations and religious ideas. The role of the State should not be to promote any one religious viewpoint, even the irreligious viewpoint: rather the main aim of the State should be to legally prevent organisations from using either coercion or deception either in the recruitment or in the keeping of adherents.
Other reading of relevance would be:
"The Subversive Puritan: Roger Williams and Freedom of Conscience" by Mostyn Roberts, 2019
"The bloudy tenent of Persecution for cause of Conscience" by Roger Williams, 1644, available here:-
https://forgottenbooks.com/it/download/TheBloudyTenentofPersecutionforCauseofConscienceDiscussed_10205908.pdf
