Atheism is, in a broad sense, the rejection of belief in the existence of deities. In a narrower sense, atheism is specifically the position that there are no deities. Most inclusively, atheism is the absence of belief that any deities exist. Atheism is contrasted with theism, which, in its most general form, is the belief that at least one deity exists.
Agnosticism is the view that the truth of metaphysical and religious claims such as whether God, the divine or the supernatural exist are unknown and perhaps unknowable. In the popular sense of the term, an agnostic is someone who neither believes nor disbelieves in God, whereas an atheist disbelieves in God. Agnosticism is a doctrine or set of tenets rather than a religion as such.
Religious skepticism questions religious authority and is not necessarily anti-religious but is skeptical of a specific or all religious beliefs and/or practices. Modern religious skepticism typically emphasizes scientific and historical methods or evidence, it 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. Religious skepticism is not the same as atheism or agnosticism, though these often do involve skeptical attitudes toward religion and philosophical theology (for example, towards divine omnipotence). Religious people are generally skeptical about claims of other religions, at least when the two denominations conflict concerning some stated belief. Additionally, they may also be skeptical of the claims made by atheists. The historian Will Durant writes that Plato was “as skeptical of atheism as of any other dogma.”
I thought it might be interesting, even ironic, to read here what the Bible says about various forms of non-belief. But I restrained myself because it is generally unflattering. I will say only this:
For God so loved the world that he gave his only Son, that whoever believes in him shall not perish but have eternal life. [John 3:16]