But the Levites after the tribe of their fathers were not numbered among them.
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George Leo Haydock
AD 1849
Levites. As they attended the tabernacle, like God's peculiar servants, and were not obliged to go forth to battle, it was not necessary to number them with the rest. (Calmet)
They might, however, fight if they thought proper, as the Machabees did. See Josephus, Antiquities iii. 11 and iv. 4. (Tirinus)