adjust an oven rack to middle position and heat the oven to 350°. Grease an 8/2 by 4 1/2 inch loaf pan and set aside.
Whisk the flours, sugar, and salt together in a bowl. Whisk the buttermilk, molasses, and egg together in another bowl. Gradually stir the buttermilk mixture into the flour mixture until mixed. Stir in the raisins. Stir the baking soda into the boiling water until dissolved, then stir into the batter until just incorporated.
Scrape the batter into the prepared pan and smooth the top. Bake until a skewer inserted into the center of the loaf comes out clean, 50 to 60 minutes. Rotate the pan halfway through.
Cool the loaf in the pan for 10 minutes. Run a small knife around edge of the pan to loosen, then turn out onto a wire rack and cool for one hour before serving. Makes one eight inch loaf