My cagey wording was an acknowledgement that the Japanese path to secularisation is likely different than western or historically Christian countries. Christian heritage, in particular monotheism, is one component of many secularisation models, but there are other elements which apply to Japan such as industrialisation, urbanisation, differentiation, individualism, and relativism. (Steve Bruce’s model) Other models emphasise existential security and its inverse relationship with religiosity. (Pippa Norris and Ronald Inglehart) Traditional Japan would be considered a religious society from a sociological perspective, though different in form from Christian cultures. Religion is often measured by sociologists using three criteria: membership, practice, and belief. All these criteria likely take different forms in Japan, but what they will share in common with western countries is that all three measures likely show a downward trend in recent decades. This is how secularisation is generally defined and established to have taken place and or in the process of taking place.
https://scispace.com/pdf/secularisation-r-i-p-nonsense-the-rush-hour-away-from-the-4dg611cwzp.pdf