The ring was a family crest.
"His father ordered his servants, ‘... put a ring on his finger.'" (Luke 15:22)
Jewish men were not in the custom of wearing jewelry for decorative fashions. The ring the father gave the son was a signet ring bearing the family crest. Thus, by allowing the prodigal son to wear this ring, the father is inviting him into his home as part of his family.+
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