Secular Jew is a fairly Israel specific term. Secular Christian and Secular Muslims are not terms that people use at all. Christianity and Islam are religions first and foremost, they are not really an ethnicity/tribe/nation/people in the same way Jews are. Most of the time, they are religious identities that are separate from your national/tribal/ethnic/cultural identity. That also means that when you stop believing in Christianity, you simply stop being a Christian. I think a better way to phrase it would be Nominal, non-practicing, or cultural Christian, which are terms that are used. Secular Christian sounds like an absolute contradiction to us. I say all this as a former Christian who has lived in a Christian society my entire life. I am not Muslim but I assume the same is true for Islam.