some jackass was one day sprouting off in Crooklyn HQ. He was relating some highly convaluted story that no one else would believe to some fellow bethelites. "Its the Truth! Its the Truth" he yelled out. Some visitors passing by with the tour guide heard this amazing declaration. They rushed home to their congregations and relayed what they heard. And the message was relayed throughout the ranks of the JWs like wildfire. Eventually this new idea got back to the head honchos at Crooklyn and they began incorporating it into their propoganda.
And that is how the expression 'The Truth' came about.