This statement never made any sense to me until now.
During a time in your life that is physically very difficult- when you’re working hard and sleeping less and eating poorly or not at all- it is more important to be clean and sanitary than at any other time. This is because hygiene is primarily about avoiding diseases and infections, and you are more likely to suffer from these at times when your body is worn down and drawing on its last energy reserves. The body is unable to suppress even relatively weak viruses.
In the second volume of The Gulag Archipelago, Solzhenitsyn talks about a very strange, very rare sort of person he encountered in the camps who categorically refused to assent to any small erosions of the soul which the camps were designed to facilitate. In particular, he observes a man named Grigori Ivanoff (I think?) who refused even to steal any of the potatoes he was harvesting, despite his starvation and the prison camp culture of thieves, stool pigeons, and gangsters doing “whatever it takes”. The most innocent were always punished the most severely and cruelly, presumably because the zeitgeist of the endeavor was to destroy innocents in mass.
During his time at the camps, Ivanoff consistently chose the harder tasks and greater privations because he refused to compromise in matters of the conscience, which was seemingly the only way of preserving the body. And yet, he seemed to actually get stronger with time, as if his tiny bread ration had something extra in it. Solzhenitsyn establishes a pattern of such strange, phenomenal people with a few more examples, and wonders at it.
I believe the reason is the same as in the case of hygiene, except we replace “health” with “conscience”, “hygiene” with “absolute honesty and integrity”, and “physical trials” with “trials for the soul”. This seems to be because degrading the conscience eventually degrades the body, by adding strain to it in the form of cognitive dissonance, confusion, anger, and internal anguish. This phenomenon is not necessarily mystical in character- if a starving prisoner eats a child to save himself (many children were eventually born and raised within the camps, and became feral for lack of exposure to civilization), the nourishment may grant him an extra week at best. But the stain on his conscience has a real, appreciable cost. At difficult times, this additional strain will wear the murderer’s body down, perhaps pushing him over the edge into death.
And this is why we say cleanliness is next to Godliness. They are analogous. In fact, the only exception I can suppose between their perfect similarity is possibly in the relation of the soul to the body. If the soul leaves, the body dies. But if the body dies, the soul continues.