Since the 1960s, the Orthodox world has been justifiably proud of the ba'al teshuvah movement - the large numbers of assimilated Jews who have become frum, bolstering the observant community.
Until relatively recently, however, the flow of people moving the other way, out of Orthodoxy, has been the movement's dirty little secret.
True, over the past decade there has been a growing number of parents expressing public despair that their children were going "off the derech" - that is, off the path of the Torah, and seeking help in returning them to the fold.
But there has been little acknowledgement of the impact this has had on Orthodoxy as a whole - even though, according to some informal estimates, there are as many Orthodox people dropping out as ba'alei teshuvah dropping in -and little interest in what happens to these youngsters once they have become secular, beyond their impact on their families' dynamics.
A new book is set to change all that, at least in the Israeli context. Hadatlashim - a slang Hebrew acronym that stands for hadati'im leshe'avar, or "the formerly religious" -- by Poriya Gal Gatz, charts the inner lives of Israelis who have abandoned tradition, and examines what they have in common.
Gal Gatz, granddaughter of a former rabbi of the Kotel who abandoned religion in her 20s, comes to the startling conclusion that, unlike in past generations, when Israelis who left Orthodoxy simply became secular, today's drop-outs form a distinct and in many ways unique sector in Israeli society.
While they are by no means practising Jews, and certainly do not identify as such, their religious education has left an indelible impression on them. They never quite shake the language and world-view of the Orthodox Jew and often say they can identify another datlash as soon as he or she opens his or her mouth. They retain some religious habits -- anything from an intensive Seder night to saying a blessing after using the toilet - and often hang-ups, particularly to do with modesty or sex (avoiding, for example, dancing in nightclubs).
They are often happiest socialising with each other; dating sites exist that cater only to datlashim. Politically, even some of those who move to the left cannot see the right as "the enemy", and events such as the 2005 exit from Gaza cause genuine emotional turmoil.
In short, they check out of institutional Orthodoxy, but can never really leave.
It is tempting to see the emergence of the formerly Orthodox as an identifiable sub-sector as an unmitigated tragedy for Orthodoxy. And certainly serious questions need to be asked about why so many youngsters seem to be rejecting it, particularly as Israel's religious youth receive the best Jewish education in the world, and undergo an intensive experience of Orthodoxy and Jewish life.
While the reasons for leaving Orthodoxy are individual and varied (and relatively rarely concern theological questions), one frequent thread is a rebellion against an increasingly dogmatic environment, in which questions are not encouraged and in which ever more rigid standards of religious observance are required.
As similar trends seem to be occurring in the diaspora, with mainstream Orthodoxy drifting ever further to the right, there are lessons for us here, too. And yet, I cannot help but see in the datlashim a sign of hope. One of the most worrying aspects of Israeli society is the outright hostility between its Orthodox and secular components. Thanks in large part to the partnership of synagogue and state, too many secular people see religion as an outdated anachronism forced on them unwillingly, and religious people as work-shy, army-shirking parasites.
Too many Orthodox people see the secular majority as value-less. The lines between the two camps are very starkly drawn; unlike in the diaspora, secular Jews are not shul-goers, the educational systems are completely separate and, increasingly, they live in separate neighbourhoods and even separate towns. (A major exception are secular Sephardim, who tend to be quite religiously traditional.) Politics, too, tends to fall along religious lines.
So while I mourn every Jew who abandons religion, it is heartening to see a group that can potentially cross bridges; that has genuine sympathies with, and ties to, both groups. Israel desperately needs to mend its religious rift. With time, this disaster for the Orthodox camp may yet turn out to be a blessing for the nation.