James' Catholicism became public knowledge in 1669 and was furthered when he refused to swear loyalty to the church in 1673 - violating the Test Act
Charles was only three years older than James and there was therefore hope he would die before Charles did, leaving his protestant daughters, Mary and Anne, to be heirs
Titus Oates came up with the popish plot in 1678, saying that there was a plan to assassinate Charles and replace him with James. It was believed France and Ireland would support this due to the expansion of France under LouisXIV
The test act of 1678 excluded Catholics from parliament.
Charles dissolved the cavalier parliament in January 1679 and the First Exclusion Parliament met in March 1679 as Charles needed parliament for money
The main concerns of the exclusion parliament was the safety of parliament and the subject under a catholic ruler