After leading the reader through most of the major JW doctrines, the Watchtower Society's introductory book, What Does the Bible Really Teach?, really starts laying out the reasoning that you HAVE to be a JW to be approved by God in chapter 15. It is interesting how the first instruction given here is not to go investigating what other religions teach. But, as the WTS writers often do, they resort to circular reasoning and false analogy to support this:
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bh chap. 15 pp. 144-145 pars. 2-4 Worship That God Approves ***2
How can you know the right way to worship Jehovah? You do not have to study and compare the teachings of all the many religions. You need only learn what the Bible really teaches about true worship. To illustrate: In many lands, there is a problem with counterfeit money. If you were given the job of picking out such false money, how would you go about it? By memorizing every kind of counterfeit? No. Your time would be better spent if you studied real money. After you knew what real money looked like, you could recognize a counterfeit. Similarly, when we learn how to identify the true religion, we can recognize those religions that are false.The counterfeit money analogy is flawed because it assumes that one knows what real money looks like in the first place.
A better analogy would be if you had a set of imaging data of Mars' surface, but based on that data ten scientists each have have a different interpretation of what geological features are indicated by this data. Would it be adviseable to become convinced that the reasoning of scientist #1 was rock-solid, declare it is "what the data really teaches", and shun all other scientists for not agreeing without even looking at their research?
The rest of the section deals with the consequence of going with the "counterfeit" version rather than the "real" one:
3 It is important that we worship Jehovah in the way that he approves. Many people believe that all religions are pleasing to God, but the Bible does not teach that. It is not even enough just to claim to be a Christian. Jesus said: "Not everyone saying to me, ‘Lord, Lord,’ will enter into the kingdom of the heavens, but the one doing the will of my Father who is in the heavens will." To have God’s approval, therefore, we must learn what God requires of us and do it. Jesus called those who do not do God’s will "workers of lawlessness." (Matthew 7:21-23) Like counterfeit money, false religion has no real value. Even worse, such religion is actually harmful.
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Jehovah gives everyone on earth the opportunity to gain everlasting life. To have eternal life in Paradise, however, we must worship God properly and live now in a way that is acceptable to him. Sadly, many refuse to do so. That is why Jesus said: "Go in through the narrow gate; because broad and spacious is the road leading off into destruction, and many are the ones going in through it; whereas narrow is the gate and cramped the road leading off into life, and few are the ones finding it." (Matthew 7:13, 14) True religion leads to everlasting life. False religion leads to destruction. Jehovah does not want any human to be destroyed, and that is why he is giving people everywhere an opportunity to learn about him. (2 Peter 3:9) Really, then, the way we worship God means either life or death for us.