In the last portion of Hobbes' Leviathan, he outlines how abuses of religion and philosophy have led to what he calls the Kingdom of Darkness. This is not Hell in the way it is sometimes understood, mainly, as a place of eternal torment to which the souls of sinners are sent. Instead, it "is nothing else but a confederacy of deceivers, that to obtain dominion over men in the present world, endeavor by dark, and erroneous Doctrines, to extinguish in them the Light, both of Nature, and of the Gospell; and so to dis-prepare them for the Kingdom of God to come." There are four causes of spiritual darkness: 1) misinterpreting scripture; 2) introducing false doctrines of demonology; 3) mixing scripture with "the vain and erroneous Philosophy of the Greeks, especially of Aristotle;" and 4) unfairly incorporating history and tradition into philosophy and religion.
This first cause of darkness, the "main abuse of scripture, and to which almost all the rest are either consequent, or subservient," is saying that Kingdom of God is present in this world. As Hobbes noted in the previous book, what follows from this is the belief that the Church, and especially Popes, should rule over men, which serves only to undermine the authority of the sovereign. Also following from this is the mistaken belief that ceremonies and consecrations actually take place with God present, a belief which religious authorities abuse by claiming to be a privileged conduit to God. During ceremonies such as baptism, communion, and marriage, God is not present, and is not somehow brought into those partaking in the rituals due to the actions of those performing them.
In Hobbes' discussion of this first cause he elaborates on his conception of the afterlife. He argues that scripture does not justify the belief in an immortal soul that is sent either upwards to heaven for reward or downwards to hell for punishment. As Milner notes, "eternal life does not consist in such immortality but in the resurrection of the bodily life of man on earth - a life that shall be endless in the finally established Kingdom of God. The alternative fate...is not an everlasting torment but an everlasting death, that is, simple annihilation." In other words, what is thought of as heaven is actually the earth we currently inhabit - with the obvious addition of God's presence establishing it as his Kingdom - and those who are said to be 'sent to hell' simply do not enjoy this Kingdom of God. With regards to hell, Hobbes even goes so far as to doubt whether God, "the father or all mercies," would subject sinners "with all the extreminity of torture that man can imagine, and more."
The Second cause of darkness, the doctrine of demons and spirits, is liable to be abused by those wishing to manipulate the fears and anxieties of others. Hobbes says that while scripture does say there are angels and spirits, both good and evil, it does not say there are incorporeal ones that men see in dreams or visions, or corporal ones that inhabit men's bodies and posses them. If we realize that the senses are caused by impressions from the external, not by some incorporeal cause, we are less apt to be taken by these insidious doctrines.
As noted before, Hobbes' philosophy often directly opposes that of Aristotle. This third cause of darkness, mixing Greek philosophy with religion, is yet another instance of this opposition. In a particularly harsh condemnation of Aristotle, Hobbes claims that "scarce any thing can be more absurdly said in naturall Philosophy, than that which now is called Aristotle's Metaphysiques; nor more repugnant to Government, than much of that hee hath said in his Politiques; nor more ignorantly, than a great part of his Ethiques." One might ask what this has to do with the Kingdom of Darkness, but Hobbes believes that Aristotle's philosophy, particularly his essentialism, provides justification for false beliefs like a man's eternal spirit (his soul is his "essence"), and spawns an entire branch of "supernatural philosophy." Particularly harmful is the implication that qualities (or "essences") like faith and obedience become somehow separated from man rather than being of them, which is liable to abused by others at the expense of the strength of the commonwealth.
Lastly, Hobbes argues that arguments based on history and tradition poison both philosophy and religion, as they give justifications to false doctrines at the expense of reason. Things are called heresy if they do not conform to past ideas, and one becomes stuck in the Kingdom of Darkness because certain authorities benefit from it. In a not-so-subtle criticism of Galileo's execution, Hobbes faults the "authority ecclesiastical," asking of the punishment, "What reason is there for it? Is it because such opinions are contrary to true religion? That cannot be, if they are true." He also makes a case for his own work, urging the readers and his contemporaries to judge it on its own merits, and not to disavow it simply because it is contrary to their opinions.
Hobbes' discussion of religion is often accused of being overly anti-Catholic. Particularly in this book, Hobbes attacks many of the main tenets of Catholicism, in particular the doctrine of transubstantiation - the belief that upon blessing the communion wafer is literally turned into the body of Christ. Not only did this imply the Kingdom of God existed in the natural world, it also gave priests the divine-like power of summoning Jesus into this world. In addition, Hobbes attacked the canonizing of saints, and the concept of the Pope being "Christ's vicar on earth."
One can speculate as to why Hobbes launched such a vicious attack on Catholicism. At the time of its publication Hobbes was living in exile in France, and might have hoped this could curry favor in his home of Protestant England. As an avid historian, he might have been reacting to the abuses of the Church that characterized the Dark Ages, such as the corrupt selling of graces and the general suppression of non-religious thought. While both of these reasons have their justification, taking them as the sole reason for Book IV unjustly severs it from Hobbes' overall project of Leviathan.
To begin with, Hobbes attacked not just Catholicism, but the abuse of religion by any faith in general. After devoting Book I and II to showing how a proper state should function, and Book III to assuring the reader that this did not contradict Christianity, he then turns to manner in which the state has been abused and co-opted by self-interested parties. Often times these parties have been religious, since as Hobbes said, fear is the seed of religion, and this type of fear can be easily manipulated, especially with the aide of (misinterpreting) scripture. But these need not be religious authorities, as his lengthy attack on Aristotle shows. Generally speaking, Hobbes argues that any such mis-government is detrimental to the state and its denizens.
Hobbes' choice of language, "Kingdom of Darkness," may again be a way to connect with his readers. By describing an imperfect state of affairs in such stark religious terms, Hobbes (whether consciously or not) presents the choice between living in a commonwealth as he has described it and the alternative status quo. Darkness implies not only irreligion, but lack of reason. Hobbes argues, inevitably, that to resist his theory of society is to resist reason.
Thus Leviathan overall, and Book IV in particularly, appeals to the Enlightenment theme of reason as a liberator of humankind. Surely he hoped the reasoning contained in his tome would liberate his contemporaries from their present warring condition. Hobbes certainly knew his work would meet with disapproval, not just from religious authorities but people opposed to the idea of an absolute monarchy, and pleaded "there is nothing in this whole Discourse...contrary either to the Word of God, or to good Manners; or to the disturbance of the Publique Tranquility," and that "such Truth, as opposeth no mans profit, nor pleasure, is to all men welcome." That fact that we read it over 300 years later, even when the idea of an absolute monarchy has been long discounted, shows that to some extent, Hobbes got what he desired.