It recalls Gideon’s meeting with the angel who came to commission him as deliverer under the terebinth
(Judges 6:11). This parallel is designed to show the difference between the two appointments. While Gideon builds an altar to God instead of the terebinth when he is commissioned to deliver Israel, Abimelech receives his office at the terebinth with its idolatrous connotations.
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6 And all the leaders of Shechem came together, and all Beth-millo, and they went and made Abimelech king, by the oak of the pillar at Shechem.