The First Estate was one of the three dividing bodies in France that had originated in the Middle Ages. It consisted of members of the clergy, all of which were exempt from taxes before the Revolution. Tax exemption was one of the various privileges that members of the First Estate appreciated; so many were against a reform that would lessen their privileges. The First Estate had considerable power when an Estates-General was called, as it almost always agreed with the Second Estate, each giving one vote to beat the usually contradictory single vote of the Third Estate.
The financial status of members of the First Estate varied greatly. Those higher in the ranks of the Catholic Church had a very large income, while others could have been considered destitute. Many of the destitute members were sympathetic to the concerns of the Third Estate during the reign of Louis XVI because they had family in the Third Estate.
Most information from lectures by professor Storch at the University of Wisconsin - Rock County. Additional information from Fourth Edition Western Civilization from 1300 by Jackson J. Spielvogel.