To assess whether or not Jehovah’s Witnesses are preaching the same gospel as Paul preached, we must examine a number of factors including:
a. gospel content
b. nature of salvation
c. means of and criteria for salvation
d. hope offered by the gospel
Let’s look at content first.
Here is what Paul says to the Galatians in chapter 1:6–9 NWT
I am amazed that you are so quickly turning away from the One who called you with Christ’s undeserved kindness to another sort of good news. 7. Not that there is another good news; but there are certain ones who are causing you trouble and wanting to distort the good news about the Christ. 8. However, even if we or an angel out of heaven were to declare to you as good news something beyond the good news we declared to you, let him be accursed. 9. As we have said before, I now say again, Whoever is declaring to you as good news something beyond what you accepted, let him be accursed. Gal.1:6-9 NWT
But according to the Watchtower good news we find:
What Now Distinguishes the Good News to Be Preached? What vital thing has now been added to the good news of the kingdom of God that Jesus Christ and his zealous apostles used to preach nineteen hundred years ago? This, namely, the birth of God’s Messianic kingdom in the heavens at the end of the Gentile Times in 1914. And after the ensuing war in the invisible heavens and the ouster of Satan the Devil and his demons from heaven and down to earth, this announcement was sounded forth, to be added to the good news, as stated in Revelation 12:9-12 WT 67 12/15 pp. 758
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“Note, however, that the message that Jesus said would be proclaimed in our day goes beyond what his followers preached in the first century.” Benefit From Theocratic Ministry School Education page 279:
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"That issue of The Watch Tower further stated: "in 1920,...Bible Students came to a correct understanding of our Lord's prophecy contained in Matthew 24:14. They then realized that 'this gospel' which was to be preached in all the world for a testimony to the Gentiles or all nations, was not a gospel of a kingdom yet to come but a gospel to the effect that the Messianic King has begun his reign over the earth. "WT Feb 1, 2006 p.25
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If God Has an Organization, What Is It? Let the honest-hearted person compare the kind of preaching of the gospel of the Kingdom done by the religious systems of Christendom during all the centuries with that done by Jehovah’s Witnesses since the end of World War I in 1918. They are not one and the same kind. That of Jehovah’s Witnesses is really “gospel,” or “good news,” as of God’s heavenly kingdom that was established by the enthronement of his Son Jesus Christ at the end of the Gentile Times in 1914. (Luke 21:24) WT 1981 5/1 pp.17-18
Conclusion: The Watchtower is not preaching the same gospel the Apostles preached in the first century; instead they admit that they go beyond what Paul wrote and add extra content to the gospel. So what did they add?