WASHINGTON — Thousands of people streamed onto the National Mall for a daylong prayer rally Sunday billed as a “rededication of our country as One Nation under God.”Against the backdrop of the Washington Monument, worship music blared from a stage that made clear the event’s Christian focus. Arched stained-glass windows, set underneath grand columns resembling a federal building, depicted the nation’s founders alongside a white cross. Most speakers celebrated Christianity’s ties to American history, a blending of ideas that critics flagged ahead of the prayer gathering as supporting Christian nationalism. From the stage, the Rev. Robert Jeffress embraced the term,...