"Jew" is an ethnic term as well as referring to a member of a particular religion. So a "secular Jew" is someone who is ethnically Jewish but does not follow the religion of Judaism (and usually DOES follow some cultural traditions).
"Christian" does not refer to an ethnic group so there is no real equivalent. Anyone who does not follow the Christian religion is not a Christian ("secular" or otherwise) even if they do keep some cultural traditions derived from Christianity.