Sincerity is, like respect, something earned. You haven't earned it.

If you're sincere or not is a function of your character.  It's not something you just turn on or off.  I know the term's a big buzzword for your cult leader but what it means is really rather simple.  It means honest. 

So I can't help it if you're not honest with me.  I'm honest with you.  I believe everything I say to you.  I might be wrong about this or that, you might not like what I say about this or that but the fact is, I mean what I say. 

You, on the other hand, are suggesting that you're not honest in the same way.  Not much I can do with that except warn others, where I see fit, that you openly admit that you can't be trusted.  You might choose to be honest with one person and not with another.  It all depends on whether or not you think they're "earned" your honesty. 

Thanks for the warning.