Sorry, I have to put it so bluntly. No serious historian could accept the validity of George Washington's "The Daily Sacrifice" Prayer Journal. Handwriting experts have proven that the journal was not written in his own hand. And even pietists Peters Lillback and Marshall refuse to endorse it.
Brad Hart pointed to a very valuable source that irrefutably proves the journal a fraud. From that source, Frank E. Grizzard's assessment in an email he sent to Ed Brayton:
"The so-called prayer journal is not in GW's writing, although I'm not sure it's actually a forgery. The manuscript dealer (Burk I think) who first sold it when it came to light in the 19th century printed a facsimile edition in which he admits that the Smithsonian rejected it as a non-GW document, but it did have Washington family provenance, so he said. Thus it apparently was a descendant's. Johnson's version is taken from Burk. The prayers are based on the English prayer book."
-- Frank Grizzard (senior associate editor of the George Washington Papers collection at University of Virginia) [in an email he sent] to Ed Brayton (2004).