The Institutes, published as part of a series of works on religious education, was "a summary of a half-century of the writing of liberal theologians on a number of issues and was to become a standard exposition of beliefs for generations of Unitarians.

However, researching and writing the work eventually convinced Priestley to abandon the Calvinism of his youth and adopt Socinianism.

In an effort to increase and stabilise membership at his church there, he taught three religious education classes, all outlined in his text.

[5] Unlike the later Sunday schools established by Robert Raikes, Priestley aimed his classes at middle-class Rational Dissenters; he wanted to teach them "the principles of natural religion and the evidences and doctrine of revelation in a regular and systematic course", something that their parents could not provide.

[9] The Institutes shocked and appalled many readers, primarily because it challenged basic Christian orthodoxies, such as the divinity of Christ and the miracle of the Virgin Birth.