This verse is particularly difficult to fully understand, because Benjamin makes assumptions of his listeners that are beyond most of us. The pieces of the puzzle are:
There are two readings for this verse, and it is likely that Benjamin intended both of them to be present in the minds of his audience. The first is the clear separation between good and evil. The good are with God, the evil are expelled. In the same way that the evil/unclean ass is expelled from among the “clean,” so to are those who do not know the name of Christ expelled from the presence of those who do know the name by which they are called (these latter being in the presence of God).
The second reading is more social. Just as the unclean animal must be expelled from the clean flock, so too in Benjamin’s land must the “unclean” person – the one who does not take upon himself the name of Christ – be expelled. It will be remembered that Benjamin has no police force to ensure social harmony. Benjamin is indicating a spiritual rejection by the heavenly king, and possibly a physical banishment by the earthly king.