Thank you for the biblical question on Numbers 31:18 as it applies to Christianity.
The citation of Numbers 31:18 does NOT apply to Christianity. That command of God was given to the nation of Israel in their conquest of the Promised Land.
It would be equally mistaken to think that Christians have to bring an animal sacrifice for their sins or that all Christians have to gather in Israel to celebrate the Day of Atonement or the festivals required by God. No one would think that all Christians have to live in the Promised Land.
Christians are not commanded to kill people. The interpretation and application of Numbers 31:18 regarding Christians are false and faulty.
SUMMARY: Proper interpretation and application must be considered in the context of the passage.
-Kingston