what is its religious significance?
It was used in Brahmanic sacrifices, ancient Egyptians, Greeks and Romans invoked their gods with offerings of salt and water, Huixtochihuatl in Aztec mythology was a fertility goddess who presided over salt and water, it is also considered to be a very auspicious substance in Hinduism etc
Salt
Brahmanic sacrifices, in Hittite rituals and during festivals 
where does salt come from in the world Sodium chloride and seawater how is it produced? Salt is extracted from the vast sedimentary deposits  and is then mined directly or extracted and then purified by mechanical evaporation of brine. It is then refined and goes through recrystallization.
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Salt [SEP] In religion
Salt has long held an important place in religion and culture. At the time of Brahmanic sacrifices, in Hittite rituals and during festivals held by Semites and Greeks at the time of the new moon, salt was thrown into a fire where it produced crackling noises. The ancient Egyptians, Greeks and Romans invoked their gods with offerings of salt and water and some people think this to be the origin of Holy Water in the Christian faith. In Aztec mythology, Huixtocihuatl was a fertility goddess who presided over salt and salt water. Salt is considered to be a very auspicious substance in Hinduism and is used in particular religious ceremonies like house-warmings and weddings.