Q. Why are so many people, even settled adults, dropping out of church? I recall a passage from 1 John which implies that those who remained are part of those who are saved, but those who depart were never really part of the kingdom. What is your take on this issue?
An excellent principle of biblical interpretation is that we first need to understand what a Scriptural passage was saying to “them then” before we can appreciate what it means to “us now.” (This principle is articulated in the book How to Read the Bible for All Its Worth by Gordon Fee and Douglas Stuart.)
I believe this is the passage you are referring to in 1 John: “They went out from us, but they did not really belong to us. For if they had belonged to us, they would have remained with us; but their going showed that none of them belonged to us.”
When we investigate the meaning of this statement in light of the original context of 1 John—its historical setting and the reason why it was written—we discover that those who “went out” are people who left the Christian community that John was addressing because they chose to follow a false teaching: that Jesus hadn’t come to earth in a real human body. This teaching arose under the influence of the Greek idea that matter is evil and only spirit is good. The implications were that people could live in any way they wanted, since what happened in the body wasn’t important, and that there was no need to help others who lacked practical things such as food, clothing, and shelter, since those things only affected their bodies.
So what John says in this letter is that the Christians who have remained in the community, who have continued to have faith that Jesus became a real human being in order to become our Savior, shouldn’t be disturbed or shaken by the departure of many former members. Their immoral lives, lack of compassion, and denial of Christ show that they are indeed following a false teaching, and suggest that they may never have been saved in the first place.
To me this seems to be a very different case from people in Western countries (the church is actually growing vigorously in other parts of the world) not staying in church once they become adults, or never choosing to attend church in the first place. The Pew Research Center has been tracking church attendance in the U.S. for decades, and it has found that attendance has been dropping steadily with each generation. I don’t think this is due to large numbers of younger Americans leaving the church to follow false teachings. Rather, I think it reflects the cultural shift from modernity to post-modernity and the fact that the church (which tends to change its own culture more slowly) has been scrambling to catch up with that shift. I think the church needs to find a way to “speak the language” of younger generations and to express how the gospel of Jesus Christ is good news for them.
I must add that I know many younger Americans who are strong and vibrant followers of Jesus Christ. It isn’t automatic that a person in a younger generation will not find church participation meaningful or not be able to relate to the gospel as good news for them. But I do believe that the church needs to re-think itself culturally and learn to speak the new language of post-modernity if it wants to attract younger people in Western countries into the community of Jesus’ followers. This is a significant challenge, and your question points to it.
Good Question 
It could be their Dark Night of the Soul.
Some might say you aren’t really a believer if you’ve never known unbelief or true doubt. If you’re “born” into the religion (i.e. indoctrinated since childhood, before you had critical thinking ability), then you would have to go through period of unbelief before you can be called “born again.”
Churches are not important. If anything they might prevent you from connecting with God on deeper levels. Listen to what Jesus said:
“And when thou prayest, thou shalt not be as the hypocrites are: for they love to pray standing in the synagogues and in the corners of the streets, that they may be seen of men. Verily I say unto you, They have their reward.
But thou, when thou prayest, enter into thy closet, and when thou hast shut thy door, pray to thy Father which is in secret; and thy Father which seeth in secret shall reward thee openly.”
Matthew 6:5-6
That is how you you get your single eye illumimated:
“The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light.”
Matthew 6:22
Stay wise and gentle, my friends.
I think that the words of Jesus you quote from Matthew 6:5-6 cannot legitimately be used to discourage participation in a community of faith. Jesus was warning against the way some people abuse the practice of prayer to try to get admiration from others. But it does not follow from this that all prayers are hypocritical or that all practices of organized religion are to be avoided. Jesus attended the synagogue regularly and taught in the temple when he was in Jerusalem. His first followers also attended the synagogues and founded religious communities. The problem is not the practice but its abuse.