Your kid wants to join a cult

How old is the kid?

Depends on your definition of 'religion cult' because if your referring to Christianity in general it's not a religious cult in the slightest even though I am sure there have been some types that went off the rails where it's a religious cult under the guise of something good. but cults are almost always like that where they brainwash people into thinking it's good and ends up being some form of sexual abuse or some other corrupt/evil BS. but anyone who understands the Christian (especially Catholic) faith half way decently would see through that cult BS real quick.

but lets just say it was a legit cult your kid wanted to join, like say David Koresh type of stuff, naturally you would be against it. but short of using physical force to stop them, about all you could do is pray for them.

but I think that's why it's good to educate kids at a young age about Christianity so as this way if things get bad they are more likely return to it should things get bad in their lives etc since they have a foundational moral truth they can return to where as if kids lack this teaching from early on they are more likely to believe many of the worlds lies and all kinds of other shady stuff thinking it's good/okay when it's not. because if someone does not have Jesus Christ at their moral foundation it's more likely once they go off the rails that they won't return to the truth etc.

You deny Christianity cult status then go on to describe a process of indoctrination, do you not see any irony in that? All religion views other religions as "spurious" so to an extent they are describing each other as "cults".
      One thing is for sure, they certainly all started out as cults, they merely have the label stripped from them once they reach saturation level of acceptability.