Christian and Muslim states began secularizing in earnest during roughly the same historical window—the nineteenth century. The Muslim-world curve then rose faster, substantially closing the gap by the mid-twentieth century.
The data are inevitably sparse and the historical classifications coarse, but the broad pattern is clear. (The final Muslim data point is partly an artifact of many newly independent Muslim-majority states entering the dataset.)
— Metin M. Coşgel, Matthew Histen, Thomas J. Miceli & Sadullah Yıldırım, “State and Religion over Time”
https://media.economics.uconn.edu/working/2015-07.pdf