This post is written for Brandon E, but it has broader application.
The greatest truth known to mankind is that Jesus is Lord. The second greatest truth known to mankind is related to the first: that is, the Bible is the word of God. The relationship, of course, is that it is from the Bible that we can know that Jesus is Lord for it is the testimony of God through His prophets and apostles.
I can think of no truth greater than these two.