Consider the image of salt as a preservative. In the Hebrew Bible (Christian Old Testament) it was used as an offering to represent the eternal covenant between God and God's people (Leviticus 2.13, Numbers 18.19, 2 Chronicles 13.5).
As background to Jesus's birth, consider how "swaddling" involved washing a baby, rubbing it with salt and tying it in "swaddling bands". These cloths were removed periodically and the child was wiped with olive oil and dusted with myrtle.
Matthew 5.13 preserves Jesus's sharp words to his disciples. Explore why his use of salt as the metaphor was apt and what the challenge was to the disciples.
In the Jewish Talmud, salt symbolises the Torah; as the world can't exist without salt, so it can't exist without the Torah. Discuss ways in which we might use salt as a metaphor now and what 21st-century metaphors could be used by believers to represent a solemn agreement with God.