In On Councils: Their Nature and Authority, St. Robert Bellarmine answers the attack of the early Protestant Reformers on by treating on all matters pertaining to Councils.
Beginning with definitions and terms, Bellarmine explores in summary all the Councils approved in his day, as well as those only partially approved and those not approved at all. Then he examines their purpose and foundations in Scripture, the Fathers, and history.
In the second book, Bellarmine examines the authority of Councils according to the same standard, proving especially that the Pope is above Councils and is the one to summon and confirm them.
To prove his case he musters his considerable scholarship and answers not only the arguments of Luther and Calvin, but of each early Protestant to show that approved Councils do not contradict each other, and the Church does not put Councils above the Word of God.