I really like how HBO showed the awkwardness and nervousness of the principal characters present at George Washington's swearing-in as President of the United States.
When Washington adds "so help me God" to the presidential oath, those watching this scene truly understand why. It wasn't a political ploy to appeal to Christian voters - not then and not now. As courageous as Washington was in war, he was genuinely troubled by this responsibility he was undertaking. He did not feel himself adequate to the role, and was deeply concerned he would fail. And he knew that failure would not only wreck his reputation, it could also doom the nation. To steal a phrase from the movie Apollo 13: "Failure [was] not an option!" So, Washington placed his fate and that of the United States in the hand of Providence. "So help me God" was indeed an understandable thing to say.