Why it is hard to rinse off soap from your body using rain water
Rain water is pretty much devoid of the minerals such as calcium salts that are often found in so-called "hard" tap water taken from underground sources. Calcium ions in tap water react with the fatty acid anions in soap to create insoluble soap scums, which (while they may remain on your skin) do not have the slippery feel of soap. Without the calcium ions present, the soap remains in soluble form (sodium salts of fatty acids) and I would propose that the slippery feel of soap on your skin comes from the traces of it that remain even after considerable rinsing.