It was written in response to the publication of an English translation of the Histoire critique due vieux testament by the French cleric Father Richard Simon.
The tendency of Simon's book was to undermine Protestantism, which prioritises the authority of the Bible over the traditions and rituals of the Catholic Church, and so Dryden set out in Religio Laici to address the issues raised by Simon's book, along with other religious issues of his times such as Deism, in order to assert the validity of the teachings of the Church of England.
These are (in the original spelling): Dryden concludes the poem with a plea for moderation in all things including religious debate.
[1] Five years after publishing Religio Laici Dryden drastically changed his position and converted to Roman Catholicism.
The fact that he converted a few years into the rule of the Catholic king, James II, prompted much scorn and derision from contemporaries.