I wrote the Game Master's Guide, which contains a chapter on using religion in In Nomine, including variant campaigns "by the books," following the theology of Protestantism, Catholicism, Judaism, or Islam.
Some of you may be familiar with the rantings and ubiquitous comic book tracts of Jack Chick. Just for fun, I wrote a "supplement" for the GMG, on integrating Chick's theology into In Nomine.
A particularly vitriolic form of fundamentalist Christianity based on the writings of Jack Chick. Chickism is best known for the hundreds of small comic tracts distributed by Chick Publishing, which are typically left in bus stations, laundromats, libraries, and other places as a form of proselytization. "Chickism" is not truly a denomination, but merely a collection of beliefs held by the most extreme conservative branches of fundamentalist Protestantism. The most important themes are literal Biblical inerrancy and exclusive salvation through Jesus Christ. What distinguishes Chickism from "mainstream" fundamentalism is its virulent, paranoid worldview, in which virtually all expressions of popular culture and all beliefs at odds with Chickism are created and directed by Satan. Chickists believe that Satan and his demonic minions are very active in the world and manifest corporeally on a regular basis. While most fundamentalists would agree with Chick's basic tenets, that Jews, Muslims, Hindus, pagans, atheists, etc., are all going to Hell, Chickists go further and consider anyone who disagrees with them on a single point (even other fundamentalists) to be serving the Devil, wittingly or not. Chickism is particularly rabid with regards to Roman Catholicism, which Chick believes to be a powerful Satanic organization that has, Illuminati-like, infiltrated most governments and other world religions. Other "tools of the Devil" include rock and roll, fantasy and science fiction (and indeed, any work of fiction that is not explicitly Chickist), and (of course) role playing games.
Use all the rules for New Testament In Nomine (Game Master's Guide, pp.79-80), but remove any doubt as to the status of Jesus Christ; he is the Messiah, and one with God, and he now actively sits on the Throne of Heaven and personally directs the affairs of angels. There are no political conflicts in Heaven; each Archangel may have a slightly different perspective on how to serve God, but they do not argue, and all angels act with perfect knowledge of the Divine. All angels are Protestant Christians, and there are no non-Protestant souls in Heaven. All Soldiers of God must also be conservative Christian Protestants.
Demons are very active in a Jack Chick-based campaign, and can be represented much as they are in the main rulebook. The difference is that the primary goal of Satan/Lucifer (and thus, all of his minions) is leading humans away from Jesus Christ. Demon Princes may squabble amongst themselves, but they all agree that Christianity is the greatest threat to their power, and thus wield their Words in such a way as to lead humanity into other belief systems, by whatever means necessary. All religious institutions except fundamentalist Protestant Christian churches (and many of them too!) are infiltrated by demons. Obviously, Laurence is not affiliated with the Roman Catholic Church in this setting!
Playing diabolicals in a Jack Chick campaign will probably not be appealing, since it is taken for granted that in any confrontation between the infernal and the divine, the infernals will lose. Physical confrontations between angels and demons should be avoided, since angels, invested with the holy Word of Jesus Christ, are guaranteed to prevail. A Chick campaign will work best as a series of morality plays; celestials don't fight each other directly, but compete to influence individual mortals toward or away from God.
Rules changes might include granting special powers to anyone who truly accepts Jesus as his Lord and Savior. Any believing Christian may be capable of exorcising or driving away a demon just by invoking the name of Jesus in prayer, for example. Some Chickists believe that demons, even Satan himself, are simply unable to harm a Saved person, who has the protection of Jesus Christ. This would make Christians, like angels, effectively immune to any diabolical powers. If this is true, then demons would approach Christians the same way they do angels -- not by trying to harm them directly, but by trying to deceive and tempt them, since the only way to affect them is to make them lose their faith. (Alternatively, perhaps demons can physically harm a believer -- though being Saved would still make someone immune to a diabolical resonance -- but since killing a believer just sends them straight to Heaven, it's not considered productive.)
It is recommended that all angels have the Rite "Save a soul" (i.e., cause a human to accept Jesus Christ as his Lord and Savior). Likewise, all demons would have the Rite "Lead a soul to damnation" -- but the Essence is only collectible upon the human's death, since only then is his fate assured. This Rite would be worth +2 Essence if the damned human was formerly Saved.