The work, as originally planned, was to be the concluding part of his _Institutes of Natural and Revealed Religion_, but as the matter of it grew it became extended into a separate treatise, larger, indeed, than the whole of the Institutes. Its object was to show that modern Christianity was a departure from the original scheme, and that the innovations have debased its spirit and almost annihilated all the happy effects which it was eminently calculated to produce. Although it had begun to recover itself from its corrupted state, and the Reformation was advancing apace, abuses still continued in many places, even although their virulence was very generally abated and the number was greatly increased of those who were most zealous in the profession of Christianity, whose lives were the greatest ornament to it, and who hold it in such purity that if it was fairly exhibited and universally understood it could hardly fail to recommend itself to the acceptance of the whole world. “But so long as all the Christianity that is known to Heathens, Mahometans and Jews is of a corrupted and debased kind, and particularly while the profession of it is so much connected with _worldly interest_, it is no wonder that mankind in general refuse to admit it, and that they can even hardly be prevailed upon to give any attention to the evidence that is alleged in its favour. Whereas, when the system itself shall appear to be less liable to objection, it is to be hoped that they may be brought to give proper attention to it, and to the evidence on which it rests.” In this work Priestley attempted to trace every “corruption”—that is every innovation or departure from what he conceives to be the original scheme—to its proper source and “to show what circumstances in the state of things, and especially of other prevailing opinions and prejudices, made the alteration, in doctrine or practice, sufficiently natural, and the introduction and establishment of it easy.” Priestley hoped as a true rationalist that this historical method would be found to be one of the most satisfactory modes of argumentation, in order to prove that what he objected to was no part of the original scheme. “For after the clearest refutation of any particular doctrine that has been long established in Christian churches it will still be asked, how, if it be no part of the scheme, it ever came to be thought so, and to be so generally acquiesced in; and in many cases the mind will not be perfectly satisfied till such questions be answered.”