It depends on what kind/brand of soap it is. You would have to check the "Active ingredients" and contact your vet or the poison control center in your area. Most generally, it will only cause some GI upset (Vomiting &/or diarrhea) . If this happens you can always withhold it's food for 24 hours, make sure it has access to plenty of water & feed cooked white rice & low fat cottage cheese. Either together or separately, in small quantities like 1/2 to 1 cup at a time, smaller quantities for smaller dogs.
My lab eats it all the time (obviously we don't encourage it, but bite marks in the soap, missing bars, and shower-fresh breath give it away. No idea why. He's eaten probably 25 bars in his 3 years of life and he's no worse for the wear. As the other responder said, soap is relatively low on my list of worries compared to raisins, chicken bones, and other poisonous things, and it smells better than when he rolls in something.
You should get him to the vet, or at the very least call and find out what you should do, this could be toxic to his system, so I think you should really take the pup in to be seen. Hope this helps, good luck to you and the pup.
Administer a mild laxative and watch it. It should be fine, try to keep things away from it that it will eat. Get the dog a pork femur if you can, they love it.
