Here's my mega-answer. (Don't worry about all the work I put into it, I already have this stored for my own references.)
Use their own words against them:
The Truth that Leads to Eternal Life (1981) chap. 2 p. 13
Why It Is Wise to Examine Your Religion
The Bible informs us that “the true worshipers will worship the Father with spirit and truth.” (John 4:23) If our worship is to be acceptable to God, it must be firmly rooted in God’s Word of truth. Jesus reproved those persons who claimed to serve God but who relied heavily on the traditions of men in preference to God’s Word. He applied to them God’s own words from Isaiah 29:13, saying: “It is in vain that they keep worshiping me, because they teach commands of men as doctrines.” (Matthew 15:9) Since we do not want our worship to be in vain, it is important for each one of us to examine his religion.
5 We need to examine, not only what we personally believe, but also what is taught by any religious organization with which we may be associated. Are its teachings in full harmony with God’s Word, or are they based on the traditions of men? If we are lovers of the truth, there is nothing to fear from such an examination. It should be the sincere desire of every one of us to learn what God’s will is for us, and then to do it.—John 8:32.
Knowledge that Leads to Everlasting Life (1995)chap.5p.51par.20
Whose Worship Does God Accept?
With a desire to please God, each of us needs to examine our worship to make sure that it is not contaminated by ungodly practices or that we are not omitting something that he considers vital.—James 1:26.
Watchtower, May 1, 1958 p.261
Is Your Religion the Right One?
How can you tell whether your religion is the right one or not? A simple way of telling is to expose it to a test of God’s Word. If your religion is not in accord with what the Bible teaches, then it is not in harmony with truth. It is not right. “And if they speak not according to this word, they shall not have the morning light,” said God. It is important, then, that you “keep testing whether you are in the faith,” as Paul declared. Keep checking to see whether the things you believe are in keeping with God’s Word. But the question is, Are you willing to put your religion through such a test? There is nothing to fear, because if you have the right religion you can only be reassured by the examination. And if what you believe is not in keeping with the Bible, then you should welcome the truth, because it leads to light and life.—Isa. 8:20, Dy; 2 Cor. 13:5.
Watchtower, July 1, 1968 p.389
Is Your Religion the True One?
If a religion is not in accord with what the Bible teaches, then it is not in harmony with truth. It is not the true religion. (Rom. 3:4) You have nothing to fear in examining your beliefs by the standard of God’s Word, for if you have the true religion you can only be reassured. And if what you believe is not in keeping with the Bible, then you should welcome the truth, because it leads to eternal life.—John 17:3.
The following was a letter in response to an article in the December 8, 1983 Awake! entitled “The Pope on the Move.” Notice the organization’s return response.
Awake!, August 22, 1984p.28
FromOurReaders
ThePopeontheMove
You all loved it, didn’t you? You couldn’t resist taking a shot at the Catholic Church, could you? I will never forgive the way you mercilessly tore down the Pope. If Jehovah’s Witnesses must refer to cheap shots in attempting to bring down other religions, they’re in more trouble than the Catholics.
M. C., Florida
We surely were not trying to take cheap shots at the pope or the Catholic Church, nor were we criticizing Catholics.
The Catholic Church occupies a very significant position in the world and claims to be the way of salvation for hundreds of millions of people. Any organization that assumes that position should be willing to submit to scrutiny and criticism. All who criticize have the obligation to be truthful in presenting the facts and fair and objective in assessing such. In both respects we try to live up to that obligation.—ED.
The Watchtower organization claims to be the only avenue of salvation for mankind, being God’s sole visible channel for spiritual instruction. They should submit to any scrutiny and criticism, no matter the sources, just as they have stated that any organization that assumes that position should be willing. In saying “All who criticize…,” Watchtower is referring to themselves, so they “have the obligation to be truthful in presenting the facts and [be] fair and objective in assessing such.” These quotes are not written just for nonmembers. Everyone is urged to do the research and careful examinations, for their very lives are at stake.