What is Opus Dei and what role does it play in the story so far?

Opus Dei, located in New York City, is an organization that is characterized as a conservative Catholic group that has become the most wealthy, yet mysterious of the Catholic organizations. The secular media considers the group a cult. Silas works for one of the Opus Dei Bishops, Aringarosa. So, it is now established that Aringarosa, of Opus Dei, wanted Jacques killed, or at least wanted information from Jacques, but the reasons are not clear.