The phrase "ancient grudge" refers to a long-held resentment or dislike between two parties that has been going on for a very long time. Full definition
• A spike in sectarian tensions between Sunni and Shiite Muslims, where ancient grudges give rise to modern massacres. (religion.blogs.cnn.com)
«Two households, both alike in dignity, / In fair Verona, where we lay our scene, / From ancient grudge break to new mutiny, / Where civil blood makes civil hands unclean.» (selfpublishingnews.com)
All that shows is that your god is an ancient grudge - holding jerk, who can not say «I forgive you», with first getting a sacrifice. (religion.blogs.cnn.com)