Referring to the spilt salt in The Last Supper
Some people believe that spilling the salt refers to the spilt salt incident in the last supper, which was pointed out in Leonardo Da Vinci’s picture of The Last Supper.
Spilling salt means a person will shed tears
Early Norwegians and English people believed that spilling salt meant that the person will shed tears in the future to dissolve the split salt.
Salt was expensive back then
Because salt was expensive back then, spilling it meant that the person will have to pay a lot to get some more. In order to preserve the salt, people might have came up with that superstition to prevent others inside the house from spilling it.
The guardian angel spills salt to warn you of the devil
Some people believed that the guardian angel on the right shoulder spills salt in order to warn people of the presence of the devil, which usually stays behind the left shoulder. This is why people throw salt over their left shoulder when they spill salt (to hit the devil in the eye).
Salt was used in religious ceremonies
Earlier, salt was used in religious ceremonies and some people used to bring it home for good luck. When salt was spilled, it was considered bad luck.
Salt was connected to a healthy life
Because salt is used to preserve food, it was connected to longevity and health, and thus spilling it was regarded as an act that can directly affect the preservation of food.
In Germany, it was believed that spilling salt awakens the devil
Early people in Germany believed that spilling salt was an act that can awaken the devil and thus bring back luck. According to Wikipedia, a German proverb said that ‘whoever spills salt arouses enmity’. (See Why is breaking a Mirror considered bad luck)
It was believed that salt protects people from demons
Early Christians used to hang bags of salt beside babies before they were baptized. It was believed that salt can protect them from evil spirits.
Savages used to spill salt when they distrusted people
According to Wikipedia, Savages used to spill salt when they distrusted people and believed they were thieves or criminals.
Spilling salt makes the soil barren
Because salt affects the soil in a bad way by making it barren, some people might have believed that spilling salt brings bad luck.
Salt is used to make holy water
The fact that salt is sometimes used to make holy water in churches might have contributed to the belief that salt is holy and thus spilling it might have seemed like a bad omen.
It was believed that salt preserves food because it keeps bad spirits away
Some early people didn’t understand why salt preserves food so they assumed that salt keeps food fresh because it pushes away evil spirits. As a result, salt got a reputation of being holy and so spilling it was considered a bad thing.
Early people used to put salt on dead bodies to give them eternal life
Early people used to believe that by spilling salt on dead bodies they can have eternal lives. (See Why is opening an umbrella indoors considered bad luck?)