Devout, pious, religious, pietistic, sanctimonious apply mainly to persons, their acts, and their words and mean showing fervor and reverence in the practice of religion.
Devout stresses an attitude of mind or a feeling that leads one to such fervor and reverence.
Pious emphasizes rather the faithful and dutiful performance of one’s religious obligations; although often used interchangeably with devout it tends to suggest outward acts which imply faithfulness and fervor rather than, as does devout, an attitude or feeling which can only be inferred.
The term often, however, carries a hint of depreciation, sometimes of hypocrisy.
Religious may and usually does imply both devoutness and piety, but it stresses faith in a God or gods and adherence to a way of life believed in consonance with that faith.
In its basic meaning pietistic stresses the emotional rather than the intellectual aspects of religion.
Often this opposition of the emotional to the intellectual is overlooked and pietistic is used derogatorily of someone or something felt to display overly sentimental or unduly emotional piety.
Sanctimonious has entirely lost its original implication of a holy or sacred character and implies a mere pretension to or appearance of holiness or piety.