"They certainly did," Langdon said, explaining how it had taken nine years, but the Knights had finally found what they had been searching for. They took the treasure from the temple and traveled to Europe, where their influence seemed to solidify overnight. Nobody was certain whether the Knights had blackmailed the Vatican or whether the Church simply tried to buy the Knights' silence, but Pope Innocent II immediately issued an unprecedented papal bull that afforded the Knights Templar limitless power and declared them "a law unto themselves"— an autonomous army independent of all interference from kings and prelates, both religious and political.
With their new carte blanche from the Vatican, the Knights Templar expanded at a staggering rate, both in numbers and political force, amassing vast estates in over a dozen countries. They began extending credit to bankrupt royals and charging interest in return, thereby establishing modern banking and broadening their wealth and influence still further. (37.20-22)
Making someone a "law unto themselves" basically grants them unlimited power. That's, uh…pretty intense. Add the ability to accumulate vast amounts of wealth, and the Knights were pretty much unstoppable. What's interesting is that power is typically a corrupting force, so how do we know that they remained true to their original cause?