Religion is generally about seeking the truth. Whether a religious person believes only their own religion is correct or accepts the potential truth of other faiths, it can still be assumed that, at the very least, they believe their own faith to be true.
Likewise, religions are generally developed by those who actually believe in it. Dogma, liturgy and scripture are developed with the intention of reinforcing the truth as understood by believers. Those who teaching religious ideas that they themselves know to be false are generally labeled charlatans, cons and/or cult leaders (although cult leaders can also believe in their own teachings).
In contrast, a parody religion puts forth ideas that "believers" do not actually accept as true, and such people generally do not expect listeners to believe in their ideas either. Instead, their religious concepts are humorous. Sometimes the primary purpose of a parody religion is simply to make people laugh. Often, however, it is meant to highlight what followers consider silly or dangerous ideas in other religions. Parody religions are therefore satire.
The Example of PastafarianismThe Church of the Flying Spaghetti Monster, also known as Pastafarianism (a play on the word "Rastafarianism", a common term for the Rastafari Movement), is one of the more well-known parody religions. It teaches the existence of a single creator god – the Flying Spaghetti Monster – who created the world while drunk, and who continues to involve himself in the world through the touch of his noodly appendages. They finish their prayers with "Ramen" rather than "Amen."
Pastafarianism was formed in 2005 in response to arguments that Intelligent Design should be taught as an alternative to evolution in science classrooms, and the specifics of Pastafarianism generally mock Christian Creationist beliefs.
The purpose was to illustrate the subjectivity of religious claims and, thus, their inappropriateness in a science class. Just as a Pastafarian cannot possibly prove the existence of a Flying Spaghetti Monster (except perhaps by referencing their own ludicrous Pastafarian "teachings"), a Christian cannot objectively prove the existence of God or a six-day creation of the universe as held by Biblical literalists. All evidence of a Biblical creation is contained within the Bible itself.
Socially Critical or Just Insulting?Satire, which is what a parody religion is, has been a staple of Western culture for thousands of years. It is generally meant to make a point through humor. In the case of Pastafarianism, the level of ridiculousness is an integral part of the message. When we talk of religion, and religion in schools, we generally think of Christianity and sometimes other very well-known, mainstream faiths. Pastarfarianism underscores the point that if you let in one religion, constitutionally you should be letting in all of them, even the utterly absurd, because, again, you cannot objectively define a line between seriousness and absurdity.
Those directly targeted by satire, however, often find it to be less than humorous and more simply insulting. The very ridiculousness that underscores the message of Pastafarianism is taken as disrespect directed toward conservative Christianity.