Various religious entities will gain considerable power and potentially come to control societies in the new world. Non believers will persist, but might tactically blend back into the overtly religious populations. Oh, and there will be blood. Lots of blood.
A plague has swept the world killing tens, if not hundreds, of millions. Without any other details or qualifications, you can expect subtantial societal collapse. This will be extremely bad, and will kill many more millions.
Unfortunately for the world, it’s even worse: plague victims become murderous vampires. It’s not inconceivable that this, coupled with the chaos and disorder, might utterly decimate the human population beyond salvation. For the sake of discussion we’ll assume that some clusters of people can survive the first few bloody weeks.
Societal breakdown has at least two major implications that will be important throughout: governments will collapse to form local power vacuums and reliable long-distance communication will be greatly hindered. The power vacuum means that religious institutions or even cults have an opening to seize a much greater role in governance. The communication challenge will make the spread of knowledge about the vampire weakness almost mythical in nature and it will delay the understanding that all monotheistic religions can be effective.
After this faith-based vulnerability is discovered, it would quickly change the nature of the threat. It’s no longer just a disease with terrible side effects — it might just be concrete evidence of both the divine and the accuracy of religious texts. In the short term, this would likely influence many people on the fringes of a particular religion to return to the center. Christians, for instance, who slowly diverged from their youthful beliefs and religious teachings might be quick to re-embrace their religious roots.
At the same time, less religious people can still survive and thrive. Properly placed stakes and decapitation are as effective, if not more effective, than holy symbols for defeating a vampire. As a result, there is very little disadvantage for a clan of atheists or polytheists when fighting off the undead. Anyone inclined to do so could conjure a handful of plausible scientific reasons for the vampire reaction to holy symbols and the multitude of different religious symbols could be used to suggest that it’s not truly religious after all.
And therein lies the greatest potential for strife: which religion is right? It’s highly unlikely that true believers of any one religion would consider another, particularly those with radically different tenants and beliefs, to be equally right. And someone seeking to be a more faithful believer is unlikely to consider that perhaps their religion isn’t the complete picture. This conflict will make it very difficult for a single overlapping monotheistic religion to take form. Each religion will proclaim its true believers to be the most powerful, and there won’t be an objective way to measure it.
Thus, you have factions. Societal breakdown already created a power vacuum. Now you have the seeds for bloody human conflict.
For quite some time, survivors are likely to roam from place to place as scavenging will be a necessity. Even with vampires afoot, other people will be the biggest threat for a simple, time-tested reason: you cannot be certain of a person’s true intentions. However, this scenario has the potential for a twist. With rapidly increasing religiosity, communities may become more likely to embrace strangers and grow. Some religious groups might do so out of compassion. Others might choose to test strangers — perhaps using a captive vampire — and abandon or even kill weak believers.
As time passes, a few of these diverse group types will eventually settle and begin the reconstruction of society. One could imagine that settling around holy sites would be common. Resource competition and human distrust will put many of them at odds, but others might be quick to unite or ally based on their faith alone. This has the potential to cause the major new population centers to be heavily religious and massively increases the likelihood that religious leaders will govern with religious laws. Competing non-religious communities may exist, and may even be willing to be more violent to survive, but they may also be more easily outnumbered.
Now society has two fundamentally different threats: the living and the dead. Combatting the dead is arguably the easiest of the two: one cannot violate the tenants of their religion by slaying the damned. It may even be seen as a path to greater strength. Youths are likely to be very valuable weapons. Young children are naturally malleable and can easily be molded into some of the strongest believers among a society. As they grow up they would likely become the strongest warriors and may eventually overthrow the ruling class of lesser belief. Captive undead would likely be kept as a means to test and display one’s deep beliefs.
Handling the human threat is less simple, but something religiously-minded people have practiced for most of human existence. The term “infidel” is likely to make a resurgence in many societies, with those that think differently being open season if they must be killed. This will walk back several centuries of progress on tolerance, but the collapse of society probably already took care of that.
The world would probably be a violent place for a long time. The technological situation will determine the speed at which the disparate new communities grow and clash with each other. Over time the vampire threat will fade, leaving humans to fight amongst themselves. As decades and centuries pass some will start to question the religions of old. If all symbols were effective against the undead, perhaps none of them have the real truth? As a second era of globalization occurs, new religions might be founded seeking higher truth. People might start to drift toward tolerance or away from religion yet again.
And perhaps, horrified by this declining faith, one cleric might just release an old sample of the plague and begin the cycle anew.