Hi, Bells! Welcome to the board!
Wow, you've received a great deal of good advice... I agree with most of it; you'll be able to pick the advice that fits your situation and applies in the best manner, but I do want to make one qualification on Scully's excellent advice...
If I were in your shoes, I WOULDN'T use the actual TERM, "stumbled". That is a very specific word that has special meaning to Jehovah's Witnesses. If you use that in your conversations with your in-laws, that could alert them to the fact that you're carrying on conversations with - or at least learning from - [shudder!!!!] APOSTATES and APOSTATE WEB SITES!!!
Believe me, you do NOT want to go down THAT road with your in-laws!!!
However, if you can IMPLY that you've been 'stumbled' - perhaps using terms like, "discouraged", "dismayed", "upset", "disappointed", "hurt", "surprised" [not in a pleasant way], without actually using that "buzzword", you'll probably have what Scully was providing for you - a quick, easy way to get the in-laws to back off when they become too intense or intrusive or judgmental.
As to your comment:
"I've already told my bf that he needs to tell them explicitly that leaving the religion was totally his decision and I didn't influence him one bit, which he's agreed to, though to be honest I think they will always hold me in some part responsible..."
Unfortunately, his parents will probably continue to fantasize that their son has simply been "misled", as that is much less painful - and does not entail a judgment upon THEM and quite possibly "death" from "Jehovah" for failing to make certain that their son converted - than openly admitting the truth to themselves.
The truth that their son freely chose to avoid the trap of becoming a Jehovah's Witness because he innately felt that there was something not quite right about the religion - sect - cult - is going to be far, far too painful for them to face, especially since his "eternal life" is supposedly involved...
As for your question about why it is such a big deal if the Jehovah's Witnesses are a cult - well, I suspect that you ask that question due to a lack of information as to how damaging and destructive cults can be.
Shortly after I exited the cult - around the age of 30 - a very wise therapist told me to read about cults. She recommended one book; by the time I was through, I'd read around 20 books on cults. It was a real eye-opener; made me very grateful that I'd escaped...
You might want to read as much about cults as you can get your hands on. I exited a little while after "Jonestown" - you remember Jim Jones? Whose cultish brutality caused the deaths of over 900 people? Trust me, there were a LOT of books on cults written around that time - and I read them all.
Most cults tend to be mini-dictatorships; a sort of theological fascism controlled predominantly by males who believe they have a "god-given" right to rule, which makes any and all of their edicts akin to expressions coming from the mouth of "god" himself.
That is what makes them dangerous.
Zid the She-Devil