Scoring a deep cross into the surface of the dough is called "blessing the bread." Because the bread uses baking soda as a leavener, it requires minimal kneading and no rising, so you can bake it just before supper and serve it warm.
Preheat oven to 450°.
Lightly spoon flours into dry measuring cups; level with a knife. Combine flours, baking soda, and salt in a large bowl; stir with a whisk. Make a well in center of mixture. Add buttermilk to flour mixture; stir until blended (dough will be sticky). Turn dough out onto a generously floured surface; knead lightly 4 to 5 times. Shape dough into an 8-inch round loaf; place on a baking sheet coated with cooking spray. Cut a 1/4-inch-deep X in the top of the dough.
Bake at 450° for 15 minutes. Reduce oven temperature to 400° (do not remove bread from oven); bake 15 minutes or until loaf sounds hollow when tapped. Cool on a wire rack.