I wish I knew... I wish anyone knew (lol)....
I think of two groups: the THIRTY MILLION residents of the USA whom the Catholic Church says have left that denomination but not joined any other.... That's like 8% of the entire population of the country, larger than the two biggest Protestant groups combined. What FEW of those DO connect with some other church, they seem to mostly be the big, "lite" non-denoms that to ME seems like a really bad "fit" and of course any and all Pentecostal groups. To ME, by far the best "fit" would be Lutheran or Orthodox. My LCMS parish is about half ex-Catholic but I don't think that's typical, my Greek Orthodox friend indicated that a FEW but very few ex-Catholics go that way. The big, "no-theology/no-morality" non-denoms and teh Pentecosts get 'em. What are the Lutherans and Orthodox doing wrong? Why aren't WE the receivers? I can theorize but I think there is a HUGE mission field out there that Lutherans are not effectively tapping.
The other HUGE and fast growing group of course are the post-modern agnostics. ALL Christian groups are trying to reach them but while there are individual successing we obviously are losing the war - just witness everywhere in Europe or right here in California where Christianity has declined so much as to become largely irrelevant. How to much more effectively reach this group? There are a TON of suggestions and approaches, none seem to be working very well....
Among many I know, Christianity is simply an unknown - off their radar. No negative feelings toward it, just no feelings or opinions at all. Christianity is seen as something that gave grandma a lot of comfort.... Christmas is fun..... but it's just a sleepy, tiny, church on the street corner full of little gray haired widows, minding it's own business - and that's just fine. A bit too conservative when it comes to morals, a bit too quick to rebuke, but basically harmless. Hey, if it floats your boat, it's okay with them as long as you keep those old-fashioned moral ideas and those rebukes to yourself. That's pretty how how they see Christianity.... a rapidly dying hold over from another age, but typically harmless and at times a source of comfort and social life for some. GOD is another subject. They aren't opposed to God (nearly all are agnostics, not atheists - I've actually never met an atheist in person, just 2 or 3 on the internet) but they don't at all connect God with Christianity; they may speak of being "spiritual but not religious." How to reach out to them? I wish I knew....
I think in the USA anyway, people think there are two "brands" of Christianity: Roman Catholicism and Evangelicalism. Those DO tend to be the big churches in their community. The rest are either unknown or off their radar. I remember telling a co-worker (with a Ph.D.) once that I went to a Lutheran church and he asked if there were a lot of white people in my church.... after a few exchanges, he thought the Lutheran church was founded by Dr. Martin Luther King. I think Americans only know of those two options, which may be why a lot of ex-Catholics go to the Evangelical Churches. There are exceptions: Live in Utah and you know about Mormons, live in Minnosota and you know about Lutherans, live in Alabama and you know about Baptists but basically, people think there are only those two options. And they perhaps aren't happy with either.