Summary: You may be offended or disillusioned, but there is no one to turn to but Jesus Christ.
If you have been a Christian for any length of time you have seen people come and go. Some are famous people who everyone rejoices over at their conversion hoping it will influence many people only to see them convert to something else six months later or just stop talking about Christ. Sometimes it is someone you love or a friend for whom you have been praying for years. They come to church and make a profession of faith and then something the preacher says makes them turn away.
Also, if you have been a Christian for any length of time you have gone through times when circumstances have caused you to have doubts or fears. People may have hurt you or an unwise comment from the pulpit made you think about quitting church. It may have been someone speaking the truth in love about your pet sin that made you want to run.
We could speak to the parable of the seed and the sower, as many use that to explain people leaving church, and sometimes even God behind. However, they often use it to judge forgetting that the harvest is not until the end and what seems like a crop failure may ultimately yield a harvest. It may not be a hundredfold or sixtyfold, but even the good ground allows a thirtyfold.
It is heartbreaking to you, pastors and church members, to see someone who was coming so faithfully and seemed to be growing just disappear. They don't answer their phone. They never seem to be home and they changed their email as well as blocking you on FaceBook or some other social media site.
A great many people do not understand church so they do not attend for the right reasons. They have little or no knowledge of doctrine. They change churches every two years or so and may go from Left to Right and muddle in the Middle because they go for the things that are alluring for a time and when they get bored they move on. They may go because of friends or networking. That particular church may be close by or has Saturday night services or three types of services on Sunday morning; Traditional, Contemporary and Zombie Apocalypse/Emo/Goth/Vampire. Sunday School classes teach financial planning, dating skills or how to be holy in ten weeks and maybe even how to power pray in just sixty seconds a day to get God to have it your way. They think the Epistles are the wives of the Apostles and Ezekiel had a close encounter with UFOs. The band is rockin' or hip hoppin'! Maybe there are plenty of possible mates. Indeed, they go for many of the same reasons that people frequent a particular bar.
While some of the more sedate topics I mentioned have some merit to be addressed from a biblical basis it is possible that many would not like to hear what the Bible has to say on some of those issues. They would prefer the annotated and culturally relevant version that might not be as strict, judgmental, legalistic or “harsh”. True, some have gone Pharisaical in those areas, but tossing the baby out with bath water, as the cliché says, is not the proper redress of those extremes.