They teach that their religion is biblical. The truth is that every doctrine which is unique to Jehovah's Witnesses is extra-biblical. This includes their foundational doctrines which include dates, names and places not mentioned in scripture. Here is one sample of what I mean.
Did the sounding of the 7 trumpets
introduced in Revelation 8 get underway at a Bible Student convention
in Cedar Point, Ohio in 1922?
the sounding of the seven trumpets got under way in 1922, the Bible
Student convention at Cedar Point, Ohio, (Rev.8:7) featured a talk by
the president of the Watchtower Society, J. F Rutherford, based on
the scripture, ”The Kingdom of Heaven is at
Hand.” Revelation Its Grand Climax at Hand. p. 172