The ‘C1 to C6 model’, related to what is called ‘insiders movements’ within Islamic contexts, is much discussed today, for and against, in missions environments. Yet, the original starting point of this.
A brief summary of each Christ-centered community described in the spectrum
Missionaries establish a church that is basically identical to wherever they are from. Services are conducted in the language of the missionaries. They call themselves “Christians” and have very little cultural connection to the region where they plant the church.
The same as C1, except the services are conducted in the language of the region.
They have incorporated many non-religious cultural forms of the region into their community, such as dress, art, etc. They still reject any purely Islamic religious elements. They may meet in a traditional church building or in a more religiously neutral location. They call themselves “Christians” but try to have a more contextualized presence in the region.
They are similar to C3, but they incorporate some Islamic religious elements into their community – like avoiding pork, praying in a more Islamic style, using Islamic dress and employing Islamic terminology. They call themselves “Followers of Isa” or something similar. Their meetings are usually not held in traditional church buildings. They are not considered to be Muslims by the Muslim community.
They retain their legal and social identity within their Muslim community. They reject or reinterpret any part of Islamic practices and doctrine that contradict the Bible. They may or may not attend the mosque regularly, and they actively are involved in sharing their faith in Jesus with other Muslims. They may call themselves Muslims who follow Isa al-Masih, or just Muslims. They may be viewed by their community as Muslims that are a little unorthodox.
They keep their faith secret because of an extreme threat of persecution, suffering or legal retaliation. They may worship secretly in small groups. They do not normally share their faith openly and have a 100% Muslim identity.
(Source, HERE. http://www.thepeopleofthebook.org/C1-C6_Spectrum.html)
On the same People of the Book website you may also find responses to common objections to the ‘insiders’ approach. http://www.thepeopleofthebook.org/ObjectionsToTheInsiderApproach.html