I'll try to explain why soap won't ruin it. When you're seasoning your pan you heat it up to about 500 degrees f. At those temps the iron atoms start to vibrate and jostle around quite a bit. All that jostling creates little gaps in between the iron atoms that the oil you're seasoning with can seep in to. Once the oil has worked it's way into all those nooks an crannies and you let the skillet cool back down, the atoms stop jostling around as much and become tightly packed. This traps those oil molecules. They're still near the surface which gives the skillet it's non stick qualities but the oil molecules are clamped in by the iron molecules. To remove that oil you have to scratch/abrade off the surface iron.