The term "lapsed" is sometimes used to indicate that someone used to be religious (or was raised to be religious), but isn't any more. Wikipedia has an article titled Lapsed Catholic, but it's applicable to ex-non-Catholics as well
A lapsed Catholic is a baptized Catholic who is non-practicing.1 Such a person may still identify as a Catholic2 and remains a Catholic according to canon law.
The Merriam-Webster Dictionary definition of "lapsed" in relation to "lapsed Catholic" is "no longer believing or following the teachings of a religion". Lapsing is thus not necessarily connected with a lack of belief. However, Daniel Ford links being a lapsed Catholic with rejection of Catholic teaching, either totally or by being an "à la carte Catholic". Other sources associate the term with abandonment of practice of the Catholic religion rather than with rejection of its doctrine. Thus the Cambridge Advanced Learner's Dictionary defines "lapsed", again in relation to "lapsed Catholic", as "no longer involved in an activity or organization", and the Oxford Dictionary speaks only of "no longer following the rules and practices of a religion or doctrine". Richard John Neuhaus, quoting G.K. Chesterton's remark that a Protestant typically says he is a good Protestant, while a Catholic typically says he is a bad Catholic. For many, being a lapsed Catholic is just another way of being a Catholic.
and from Wiktionary:
(of a person) Changed to a less valued condition or state; especially having lost one's religious faith.
(humorous) By extension, having changed a (secular) belief or adherence.
However, the term "lapsed" doesn't directly indicate that they've failed to indicate that they're no longer an adherent.