Truth Vs Sincerity

I came across a statement from one of the ISKCON devotees on the internet
"Validity of ISKCON is proved by sincerity of its devotees".

I think this is a great logical fallacy of confusing between truth and sincerity.

That way you can be sincere and proclaim any falsehood to be the truth. A bogus argument out and out, though very tempting.

Consequences of this argument:

You come across a sincere christian-----------> Christianity is valid
You come across a sincere Muslim-----------> Islam is valid
You come across a sincere Jew-----------> Judaism is valid
You come across a sincere Atheist-----------> Atheism is valid
You come across a sincere stoic-----------> Stoicism is valid

......... Reductio Absurdum....QED.
Absurd!
Remember to weigh truth on truth scale and sincerity on sincerity scale.
(Or for that matter qualities on the quality scale)
Don't hoodwink yourself and your intellect.

All the best.