Which Truth is "the Truth"?

Jehovah's Witnesses believe that in the latter part of the 19th century Jehovah God used a young man named Charles Russell, a haberdasher, to find truth in the Bible. Russell eschewed all organised religion and started his own Bible study group, publishing the findings of his studies in a pamphlet that he paid to have printed and distributed.

In time Russell sold his business interests and eagerly began preaching the message that Jesus Christ had returned, invisibly, in 1874 [1]. He believed this was a truth revealed to him by God. He absolutely believed that Christ would end the present age with Armageddon in the year 1914. However, 1914 came and went and nothing happened other than World War 1 (then known as the Great War) started. But no Armageddon and no 'rapture'.

But Russell doggedly believed he alone had the truth. He went on to publish a series of books called "Studies in the Scriptures". Via these tomes he developed and taught his idea that through studying the Great Pyramid of Gizza, literally measuring it's passageways, man could ascertain God's divine plan for mankind down through the ages. Again, Russell believed that was truth revealed by Jehovah God. He was to claim that his "Studies in the Scriptures" was like the Bible in topical form and that only by reading his books one could fully understand God's inspired Word. In fact, if a person were to choose to only read the Bible, he would not be able to hold onto the spiritual light of truth. He actually claimed that "Studies in the Scriptures" practically was the Bible. [2].

However, what if a Jehovah's Witness in 2010 were to start preaching from the platform or from door-to-door that Christ had actually returned in 1874 and that God's plan for mankind could be ascertained from measuring a pyramid*? What if a Jehovah's Witness were to source the entire "Studies in the Scriptures" series and base their Christian beliefs on what was contained therein? Asides from believing things like Michael the Archangel is the Pope, would they be in step with "present truth"? Could they said to be in "the truth"?

You see, the question needs to be asked which truth is "the truth"? Russell thought he was in "the truth", but he obviously wasn't as Jehovah's Witnesses have rejected all of his "unique" beliefs as being false. Rutherford wasn't in "the truth" when he proclaimed that 1925 was more set in scripture than 1914. None of Rutherford's bold predictions came true. He wasn't in "the truth".

Yet Jehovah's Witnesses maintain that they are in "the truth". But how do they know? Russell thought he was, Rutherford thought he was and Witnesses in the late 80s thought they were in "the truth" when they believed the Governing Body's teaching that the generation of 1914 would still be alive when Armageddon came. Yet none of them were in "the truth"; what they believed was false.

Again, I have to ask, how do you know you're in "the truth"?

* this practise is known as pyramidology and is certainly a questionable practice for a Christian. 

[1] "Proclaimers" book pages 46-48

[2] "If the 6 volumes of 'Scripture Studies' are practically the Bible topically arranged, with Bible proof-texts given, we might not improperly name the volumes- 'The Bible' in an arranged form. That is to say, they are not merely comments on the Bible, but they are practically the Bible itself...Furthermore, not only do we find that people cannot see the divine plan in studying the Bible itself, but we see also that if anyone lays the 'Scripture Studies' ... after he has read them for 10 years-if he then lays them aside and ignores them and goes to the Bible alone...out experience shows that within 2 years he goes into darkness. On the other hand, if he has merely read the 'S.S.' with their references, and had not read a page of the Bible, as such, he would be in the light at the end of the two years." (Watchtower, Sept 15, 1910)