"has there been an article or something telling them to say this?" - Phizzy
I think this phrase became common among Jehovah's Witnesses starting in the early 1950's. The book, WHAT HAS RELIGION DONE FOR MANKIND states the following on page 17 paragraph 15: But this toleration does not mean they are obliged to put the quietus on the proclamation of the truth by which men may be turned from darkness to light, from false worship to right worship. The fact is, they are under moral obligation to their fellow men to let them hear and examine it and then say, "It is the truth." (bold mine, quotes appear in original) Now the phrase is virtually ubiquitous among Jehovah's Witnesses, a badge of honor and a way to identify fellow believers.