The number of people with a Muslim cultural background is growing in Sweden. Yet this segment of population is largely invisible in quantitative sociological research on religion and secularity. Partly, this is because survey questions probing religion are biased towards a Christian church-oriented definition of religiosity. In the sociological literature as well as in the Nordic public debate, nonetheless, there is a tendency to exaggerate the religiosity of people with a Muslim family background while down-playing the religiosity of the culturally Christian majority population. Taken together, this background justifies a new research project called “The Muslim Mainstream: Co-producing Secularity in Sweden”. This project will run from 2018 to 2021. The research project aims to answer the research questions: 1. How and to what extent are culturally Muslim people in Sweden secular? 2. In what ways is their secularity different or similar to that of their culturally Christian compatriots? 3. What are the differences and similarities between the major culturally Muslim communities in Sweden when it comes to secularity? The present presentation focus on the preliminary steps of research including empirically defining concepts for analysis and discussing preliminary results on the extent and demographically characterization of culturally Muslim people and their Christian counterparts in contemporary Sweden.