‘ And the children of Israel said, “Who is there among all the tribes of Israel who did not come up in the assembly in Yahweh?” For they had made a great oath concerning such a one as did not come up to Yahweh, to Mizpah, saying, “He shall surely be put to death.” '
Included in the oath made at Mizpah was that any who did not respond to the call of the tribal confederacy would be put to death. (These solemn oaths remind us how seriously they took their tribal covenant). Now was the time for giving account.
“In Yahweh.” Bound in covenant relationship with Yahweh.
“A great oath.” Literally ‘the great oath.' As often in Hebrew the definite article need not be seen as referring back. The great oath? Which one? The one now spoken of as having been made.
Notice the repetitiveness of the narrative. The writer has in mind that the account will be heard rather than read, and the repetitiveness ensures that the audience go along with the story. This kind of repetition is paralleled in stories among other nations, sometimes tediously.