Sigmund Freud suggested the use of soap could be used as a yardstick of civilization.  Meaning that as civilizations advanced the use of soap became more common.  

The original soaps were made from tree ash and animal fat.  Roman legend has it that soap was created when animals were sacrificed on Mount Sapo.

Rain washed water down the mountain and mixed with the fat from the animals and wood ashes into the Tiber River.  Clothes that were washed in the river were noticeably cleaner than wash done in rivers that did not contain the soapy mixture of fats and ashes.
 

Soap as we know it today most likely had it's roots in 2800 BC in Ancient Babylon.  Ancient Egyptians also bathed regularly using a substance made of vegetable oils and alkaline salts.

It was in Italy, Spain and France, however, that real luxurious soaps came into being.  In the late 6th century in Naples, soap making was a respected profession and Soap Makers were part of a guild.  Their work produced products that were considered to be by 'good workmen' as one would consider the products of a carpenter or blacksmith. 

By the beginning of the 7th century soaps using oils and lye were first produced by chemists.  By the eighth century, soap was common in France, Italy, and Spain, but was rarely used in the rest of Europe except by the very rich, until as late as the 17th century.  In 1806 William Colgate, himself a soap maker, started the first soap making business in America in New York City.

Until the American Industrial Revolution soap making was still small scale.  During WW1, with the 
shortage of fats and oils, a substitute for soaps was need.  Synthetic detergents were then invented.  Although these detergents are good to clean clothes, they are made of harsh chemicals that can and do irritate skin.  With few choices at the neighborhood store, detergents gained in popularity.  By the year 1953, the sale of detergents in the US had surpassed those of soap.  

Today true soap is still hard to find in your local grocery or drug store.  Real soap, made of fats and lye, is now mainly manufactured by small scale artisans. The techniques they use to make that soap are the same techniques used in the 7th century.

 For a step by step guide on soap making using the cold process technique and then hand-milling, check out Simply Lora's Beauty!.