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.
The model was presented for the first time in an article written by John Travis (a pseudonym), titled ‘The C1 to C6 Spectrum: A Practical Tool for Defining Six Types of ‘Christ-centered Communities’ (‘C’) Found in the Muslim Context,” published in Evangelical Missions Quarterly, in October 1998, pp. 407-408.
Here are the two pages of this foundational article: