USBN #210 August 1948 p3
Unfortunately it would seem that the knowledge "which could largely eliminate fear" has not been disclosed or identified by Baha'u'llah, so we do not know what it is. However, what Baha'u'lláh did not elaborate but what He meant by the 'world" recorded in the Crimson Book was the power of the Covenant.
The “Crimson Book" refers to the Book of His Covenant, and the reference above means the power for unity which the Covenant possesses and radiates. On page 238 of "God Passes By" you will find the crosse reference to the “Crimson Book" and the "Epistle to the Son of the Wolf."
-To Charles S. Krug, January 5, 1948, through his secretary