The organisation has spoken out about new rules put in place after the Trojan Horse extremist plot in Birmingham.
Ofsted's revised guidance for the inspection of schools now means inspectors pay closer attention to 'ensuring that schools provide a broad and balanced education for their pupils, so that young people are well prepared for life in modern Britain'.
But the Christian Institute has released details of an independent school in the Home Counties that has been told it could see its inspection rating downgraded from good to adequate because it is not adhering to the new rules.
The unnamed school has been told it must invite leaders of other faiths to speak at school assemblies.
But Simon Calvert from the Christian Institute told Premier's News Hour this is unacceptable: "Nobody can accept all faiths," he said.
"Should everybody accept Scientology? Or should we be able to say, 'Actually, we think it's profoundly wrong.'
"That's the problem with this vague, warm and fluffy wording that we're getting from the Department of Education.
"What we need is for them to go back to the drawing board and talk to the faith schools."
Education secretary Nicky Morgan brought in the new rules following attempts from a number of Birmingham schools to implement hard-line Muslim views earlier in the year.
Simon Calvert said it was a knee-jerk reaction: "The shift that's taking place here is from respecting people and understanding where they're coming from to a new approach where you actually have to respect the content of different religions.
"And, of course, if you have beliefs you can't do that."
An Ofsted spokesperson told Premier: "Inspectors will consider the effectiveness of the school's provision for pupils' spiritual, moral, social and cultural development and how the school's leadership and management ensure that the curriculum actively promotes British values.
"This includes, among other factors, pupils' acceptance and engagement of different faiths and beliefs, and their understanding and appreciation of the range of different cultures within school and further afield."