Proposed changes to marriage would open the way for Star Wars Jedi to perform ceremonies, a church has said.
The Free Church of Scotland said the Marriage and Civil Partnership (Scotland) Bill will allow groups promoting a belief to marry couples.
The government said the change was relevant to bodies such as humanists, who are classed as religious rather than non-religious at the moment.
A spokeswoman said the reputation of Scottish ceremonies would be protected.
The Free Church of Scotland has raised concerns about religious and civil partnership ceremonies being joined by a third category.
Church spokesman, the Reverend Iver Martin, told BBC Alba: "The third category is quite astonishing because it is the so-called belief category without really defining what belief means.
"There are loads of people in a diverse society like this for whom belief can mean virtually anything - the Flat Earth Society and Jedi Knights Society - who knows?
"I am not saying that we don't give place to that kind of personal belief, but when you start making allowances for marriages to be performed within those categories then you are all over the place."
The Scottish government is holding public consultation on bill.
A spokeswoman said: "Our current consultation covers not only the introduction of same sex marriage but also the detail of important protections in relation to religious bodies and celebrants, freedom of speech and education.
"As part of the consultation we have outlined the reason for suggesting a third type of ceremony.
"At the moment, marriage ceremonies by bodies such as humanists have been classed as religious, even though the beliefs of such organisations are non-religious."
She said that the bill made clear the government's determination to ensure the continued reputation of Scottish marriage ceremonies.
The spokeswoman added: "We are proposing the introduction of tests which a religious or belief body would have to meet before they could be authorised to solemnise marriage."
The Flat Earth Society encourages discussion on a theory that the Earth is a flat disk centred at the North Pole. The society also archives literature on the theory.
Founded in the 1800s by English inventor Samuel Birley Rowbotham, the society's theory is largely based on literal interpretations of passages in the Bible.
Jedi knights are characters in the Star Wars franchise, which includes books, comics, toys and films. Disney, which owns the rights to the franchise, is preparing to release new movies.
Various groups promote interest in the Jedi and include the Jedi Knight Society - which offers lessons from Master Yoda - and Temple of the Jedi Order.
Training in Jedi skills - such as use of light sabres, are also offered at a knights' school in California.