The word skeptic (sometimes sceptic) is derived from the middle French sceptique or the Latin scepticus, literally "sect of the sceptics". Its origin is in the Greek skeptikos, meaning inquiring, reflective or one that doubts. As such, religious skepticism generally refers to doubting or questioning something about religion. Although, as noted by Schellenberg the term is sometimes more generally applied to anyone that has a negative view of religion.
The earliest beginnings of religious skepticism can be traced back to Xenophanes. He critiqued popular religion of his time, particularly false conceptions of the divine that are a byproduct of the human propensity to anthropomorphize deities. He took the scripture of his time to task for painting the gods in a negative light and promoted a more rational view of religion. He was very critical of religious people privileging their belief system over others without sound reason
The majority of skeptics are agnostics and atheists but there are also a number of religious people that are skeptical of religion. The religious are generally skeptical about claims of other religions, at least when the two denominations conflict concerning some stated belief. Some philosophers put forth the sheer diversity of religion as a justification for skepticism by theists and non-theists alike. Theists are also generally skeptical of the claims put forth by atheists.
Michael Shermer wrote that religious skepticism is a process for discovering the truth rather than general non-acceptance. For this reason a religious skeptic might believe that Jesus existed while questioning claims that he was the messiah or performed miracles (see historicity of Jesus). Thomas Jefferson's The Life and Morals of Jesus of Nazareth, a literal cut and paste of the New Testament that removes anything supernatural, is a prominent example.
Modern religious skepticism
The term has morphed into one that typically emphasizes scientific and historical methods of evidence. There are some skeptics that question whether religion is a viable topic for criticism given that it doesn't require proof for belief. Others, however, insist it is as much as any other knowledge. Especially when it makes claims that contradict those made by science.
There has been much work since the late 20th century by philosophers such as Schellenburg and Moser and both have written numerous books pertaining to the topic. Much of their work has focused on defining what religion is and specifically what people are skeptical of about it. The work of others have argued for the viability of religious skepticism by appeal to higher-order evidence (evidence about our evidence and our capacities for evaluation), what some call meta-evidence.
There are still echos of early Greek skepticism in the way some current thinkers question the intellectual viability of belief in the divine.
In modern times there is a certain amount of mistrust and lack of acceptance of religious skeptics, particularly towards those that are also atheists. This is coupled with concerns many skeptics have about the government in countries, such as the U.S.A., where separation of church and state are central tenants.