Truly; I swear this is true. These assertions of veracity date from about 1900. The earliest seems to have been honest to God, which appeared in Jack London’s Valley of the Moon (1913). Goodness and Pete are probably euphemisms for “God,” which some might have considered blasphemous. Another equivalent, now considered offensive, is honest Injun (or Indian), dating from the second half of the 1800s and popularized by Mark Twain in The Adventures of Huckleberry Finn and The Adventures of Tom Sawyer.