Unfortunately this has been proven wrong again and again. The Church is slowly dying in America due to the very reasoning you exhibit above.
The church is weak in the US because churches are not teaching and preaching discipleship.
"Hey, it's all good. If something calls itself Christian, let it in!"
That is not my view and I think you know it. If you don't know that, you are in no shape to call out other people until you develop some comprehension skills.
Think about how unbiblical this approach is. God gave the Church shepherds to protect the flock. God warned of wolves in sheep's clothing. How can we harmonize that with "on just let it all in."
Since that's not my view, I can't support that perspective. But I can point out that an individual can discern whether or not there is a wolf leading their flock to destruction by their leader's attitude toward other viewpoints. If church leadership is frightened of their charges hearing other viewpoints, you can be sure that something is horribly wrong in leadership.
Currently youth are fleeing the Church at catastrophic rates. 1 in 3 to 4 of 5 are leaving depending on which surveys you believe. They're looking at the foundation and saying, "no thanks. We love our parents, but just can't buy what they're selling spiritually."
Yes. Many churches don't teach their youth people how to discern truth. If they find out that what they have been told all their lives in church is false - or that their church doesn't have an answer for viewpoints that challenge them - they may easily walk away.
Speaking for myself, I rejected the belief system that I was raised in during my second semester in college because I realized that it didn't make sense. That opposing "facts" inside the church and "facts" outside the church could not be reconciled, so I needed to figure out what was true. If there is a God (or gods), then that truth would also be true in the world. Over the course of two years, I came to believe in Christ and started the path of discipleship, but ended up rejecting much of the cultural baggage of the church that raised me.
There's no discernment in the Church anymore (generally, I know there are good discerning Churches).
There's not much discernment in churches, I agree. But discernment starts with open conversations and a willingness to confront truth.