The phrase is a propaganda tool for mind control.
The more you hear it called "the truth" and the more you call it "the truth" yourself, the less able you become to question anything the organization feeds you.
After all, it's automatically "the truth."
Independent thinking becomes dangerous, and there must be something terribly wrong with anyone who seriously contemplates leaving "the truth."
This mind control explains why whenever you confront a JW with a contradiction in WT doctrines or a disconnect between WT image and reality, their first response is a deer in the headlines stare.