Numbers 8:21

21The Levites purified themselves
The verb is the Hitpael of חָטָּא (khatta’). In this stem the meaning of the root “to sin” is likely to be connected to the noun “sin/purification” offering in a denominative sense, although some would take it as a privative usage, “to remove sin.” The idea is clear enough: They performed all the ritual in order to purify themselves ceremonially.
and washed their clothing; then Aaron presented them like a wave offering before the Lord, and Aaron made atonement for them to purify them.
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