"In the fight that ensued, Ephraim was beaten and routed. Jephthah’s men stopped them at the fords of the Jordan. When the fleeing Ephraimites tried to conceal their identity, their pronunciation gave them away. When tested by being asked to say the word “Shibboleth,” they were unable to pronounce the harsh “sh” but could only form a soft “Sibboleth.” For taking rebellious action against one whom Jehovah had appointed for their salvation, 42,000 Ephraimites lost their lives.—Jg 12:5,6"
Insight book #2 pp. 26-29
"Speech can be lifesaving or death dealing. This was shown during a conflict between the Israelite tribe of Ephraim and Judge Jephthah of Gilead. To identify Ephraimites trying to flee across the Jordan River, the Gileadites used the password “Shibboleth,” which had an initial “sh” sound. The men of Ephraim betrayed themselves to the Gileadite sentries at the fords of the Jordan by saying “Sibboleth” instead of “Shibboleth,” mispronouncing the opening sound of the word. As a result, 42,000 Ephraimites were slain! (Judges 12:5, 6) Similarly, what Christendom’s clergy teach may sound close to the pure language to those not well acquainted with Bible truth. But speaking in a false religious way will prove fatal in the day of Jehovah’s anger."
WT 1991 May 1, page 19, paragraph 17; Speak the Pure Language and Live Forever