"The "kingdom of Heaven" is a condition of the heart -- not something that comes "upon the earth" or "after death"." - Friedrich Nietzsche.
If we look at all religions that exist nowadays, they talk about the existence of a god or gods in some form or other. They are always seen as some sort of higher force looking down on humans, having some power over them. However, in the two novels, the existence of such gods is denied. Both texts were key books in promoting the idea of Existentialism. This is the belief that there is no all-powerful God and thus, people have the opportunity to do what they like in their existing lives without worrying about the consequences or results in the after life. This can be looked at in two ways; one is the belief that people have to do well on Earth because it will never get any better, or it can give people a reason to commit a lot of evil doings because they have no reason to fear a Hell in their afterlife. The former is described in the two books. They promote the idea that humans should do the best they can in their own life, thus replacing the job of any other gods.
If we look in the Bible, we see that God tried, many times to change the minds of his people but without success. However, he did have a huge power over individual people's lives and their environment. In comparison, in "The Plague," Dr Rieux tried, many times, to save the lives of the people infected with the disease but to no avail. However, he did end up with the power to gather quite a few people together in order to help contain the plague. By association, we see that a power over individuals can give someone a God-like status. This power is also evident in "Iron in the Soul," when, as much as any of the characters wanted to stop the defeat of France, they were unable to do so. However, we see that on an individual basis, people did have the power "to reverse the trend of apathy and despair" by looking inwards instead of upwards.
