The Testament contains information from the year 10,000 BC up to 1 BC (denoted in the book as the "Old World") and from 1 AD to the year 1648 (which is denoted as the "New World"). There are seven sets of the Old Testament (B.C.) and New Testament (A.D.) and designed to update itself every 100 years with the purpose of avoiding conflicts from possible interests of certain groups to acquire the Testament. The one tasked with the responsibility of recording historical events into the Testament is called the Testament Descriptor.
The Testament Descriptor is predicted to stop in the year 1648, at the time when the Peace of Westphalia should happen, and rumors circulate that this event spells the time of the end of the world.
As the duty of displaying history update cannot be placed to a human, as it will be liable in exploiting it, humanity resolved this issue by using the Leyline that governs everything as an intermediary and they successfully linked Fate with the Testament. It was proven that Fate exists back in the era of Gods. Fate basically has variability, its existence is the very reason that there is a beginning and an end to everything and together with space and time, it is accessible in the world's Leyline which governs all creation. The incessant flow of fate as long as this world exists and the Testament. As the two were linked together, adjusting with the current time, it will show the history of human until one century ahead.
The Testament, as it repeats history, was given the same position as the major religious scriptures that existed in the previous world. Except, all of those cultures were now lost because people who had resided in heaven knew that different Gods actually existed. That's why people in the Pro-Testament factions worship not the God in the three Holy books, but the Testament itself.