Sentence:  The King of Navarre suddenly abandoned his party and went over to the Catholics. The explanation of his <WSD>apostasy</WSD> was as simple as it was base: Navarre had no confidence in the success of his cause, and he cared little in his heart for anything but women and vanity. 
Meaning of the word marked with <WSD> tags:  The renunciation of a belief or set of beliefs.