Page 257 of the WT's 1965 book called "Things In Which It Is Impossible For God To Lie" quotes from the Jewish Encyclopedia (edition of 1909) regarding the Trinity doctrine. The first edition of 1901-1906 likely has the same text as the 1909 edition and the text of the 1901-1906 edition (in the article about the Trinity) can be read online at https://www.jewishencyclopedia.com/articles/14519-trinity. The WT begins its quote with the phrase "the concept" but the words replaced with "..." say "The fundamental dogma of Christianity;". The WT thus disguised the idea that according to the Jewish source the Trinity doctrine is the fundamental dogma of Christianity - not just of what the WT calls Christendom. The WT quotes the section that cites 2 Corinthians 13:14 but doesn't mention the following sentence from the Jewish source. The following sentence in the Jewish Encyclopedia is "Although the Judæo-Christian sect of the Ebionites protested against this apotheosis of Jesus ("Clementine Homilies," xvi. 15), the great mass of Gentile Christians accepted it." Granted the WT might have left out that sentence due to the WT possibly disagreeing with that sentence, but by leaving out that sentence the WT gave a very different impression to the WT's readers than what the Jewish Encyclopedia taught about early Christian belief.
Towards the bottom of page 207 of the WT's book the WT makes it sound like the Jewish Encyclopedia wrote specifically about 'the controversies between the Trinitarians of Christendom and the Jews concerning the "Trinity" ' (instead of about all Christians concerning the Trinity) and the WT begins a quote in mid sentence in that regard. But notice that the Jewish source begins the sentence with the phrase of "The controversies between the Christians and the Jews concerning the Trinity" just before saying the word "centered". That also disguises what the Jewish source said about the extent of Christian belief in the Trinity.
Before readers of WT literature accept a quote by the WT of a non-WT source as being authoritative and proving the WT's point, they should first read the context of the quote of the non-WT source.