Charles Ellicott Commentary Nehemiah 6:18

"For there were many in Judah sworn unto him, because he was the son-in-law of Shecaniah the son of Arah; and his son Jehohanan had taken the daughter of Meshullam the son of Berechiah to wife." — Nehemiah 6:18 (ASV)

Sworn to him. —Shechaniah was of the family of Arah, which had come over with Zerubbabel (Ezra 2:5). Tobiah had married his daughter, and Tobiah’s son had married a daughter of Meshullam, one of the builders of the wall (Nehemiah 3:4; Nehemiah 3:30). This family connection led to a conspiracy by oath to thwart the governor. The names of Tobiah and his son are Hebrew; and it is probable that, though naturalized Ammonites, they were of Hebrew extraction. This makes it easier to understand the ease with which the relationship was formed.