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Deuteronomy 18:20-22 — But the prophet, which shall presume to speak a word in my name, which I have not commanded him to speak, or that shall speak in the name of other gods, even that prophet shall die. And if thou say in thine heart, How shall we know the word which the LORD hath not spoken? When a prophet speaketh in the name of the LORD, if the thing follow not, nor come to pass, that [is] the thing which the LORD hath not spoken, [but] the prophet hath spoken it presumptuously: thou shalt not be afraid of him.
Guess what the Jews used to determine whether a prophet was false? If they predicted something that didn't happen and they claimed to speak for God, they were false prophets. Now the only things left to determine are (1) did Jehovah's Witnesses ever say that something would happen that didn't, (2) did they ever claim to speak in God's name as a prophet, and (3) would Jesus choose, as his representatives, people that he specifically warned AGAINST following?
"We have no doubt whatever in regard to the chronology relating to the dates of 1874, 1914, 1918, and 1925. It was on this line of reckoning that the dates 1874, 1914, and 1918 were located; and the Lord has placed the stamp of his seal upon 1914 and 1918 beyond any possibility of erasure. What further evidence do we need? Using this same measuring line....it is an easy matter to locate 1925, probably in the fall, for the beginning of the antitypical jubilee. There can be no more question about 1925 than there was about 1914." (The Watchtower. May 15, 1922. "Chronology:Stamped With God's Approval")
At one time, (in 1881) they had no doubt whatsoever about 1874, 1878 and 1914. At that time, 1925 wasn't even supposed to happen in this system, because Armageddon was supposed to have occurred in 1914.
In 1922, in the article "Chronology" cited above, under the subheading "Stamped With God's Approval" they affirmed that there was no doubt regarding the chronology of 1874. Do you know what they were saying happened in 1874, that there could be "no doubt" of? That Christ returned in that year. And they spoke it in God's name, in other words, they claimed it was God's words they were telling. They were also teaching that the world had actually ended in 1914, but that the effects would not be evidenced for some years.
They were still upholding prophecy about 1914 marking Armageddon in 1922, 8 years later. They claimed they were chosen by Christ in 1919. But, would Jesus choose those of whom he says:
Matthew 7:15-20 — Beware of false prophets, which come to you in sheep's clothing, but inwardly they are ravening wolves. Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles? Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit. A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit. Every tree that bringeth not forth good fruit is hewn down, and cast into the fire. Wherefore by their fruits ye shall know them.
If you would know those false prophets by their fruits, by whether what proceeds from them is rotten or good, what would you say of those whose fruit has proved rotten every time they have prophesied in God's name? Would Jesus choose them as his representatives?