Religion is part of the human activity that consists on practices and believes of what is considered divine or sacred. It’s of existential, moral or spiritual in essence.
Philosophy is the study of fundamental problems, such as those connected with the nature of reality, existence, knowledge and reason.
Philosophy is based on a systematical and critical approach based on rational arguments.
A scientific theory is a well sustained explanation of some aspect of the natural world on a body of facts that have been repeatedly confirmed trough observation and experiment.
String theory is more like a mathematical unified model that describes the fundamental forces of nature including quantum gravity. It predicts Einstein’s General relativity as a low energy description.
So how does string theory compares to religion and philosophy? String theory is based on a well tested body of knowledge that describes mathematically the reality of our universe (the standard model and GR). String theory is derived, and is consistent with every fundamental physical theory known to mankind. But string theory lacks direct experimental verification of its main postulates.
Is not religion, because religion is not based on scientific facts, or rational arguments? String theory is neither sacred nor divine, and also not spiritual in essence. It is not well tested to qualify as scientific full fletched theory, but is based in firmly established scientific theories.
On the other hand string theory is closely related to philosophy by the rational, systematic methods used by both disciplines, and also by their aims. But string theory is not oriented to explain why things are the way they are. It is just looking for a unified and consistent mathematical model to describe all fundamental principles of theoretical physics. String theory can not say anything about the philosophical problem of existence, for example.
So my guess is that string theory is a mathematical unified and consistent model, still at its infancy, that shares some methods and aims of natural philosophy and is fundamentaly different than relion, although it shares with religion the human need to understand creation and our place in the universe.