And after that the Levites shall go in to serve at the tent of meeting, when you have cleansed them and offered them as a wave offering. For they are wholly given to me from among the people of Israel. Instead of all who open the womb, the firstborn of all the people of Israel, I have taken them for myself. (Numbers 8:15-16)
'Wave offering' is a poor translation. As usual the NRSV has it better with 'elevation offering'. This can be defined a "ritual [that] marked a change in ownership of the sacrifice from the offerer to God".
Although offered by Israel though the Levites are already God's following the passover.
And the Lord spoke to Moses, saying, “Behold, I have taken the Levites from among the people of Israel instead of every firstborn who opens the womb among the people of Israel. The Levites shall be mine, for all the firstborn are mine. On the day that I struck down all the firstborn in the land of Egypt, I consecrated for my own all the firstborn in Israel, both of man and of beast. They shall be mine: I am the Lord.” (Numbers 3:11-13)
Similarly the church offers the sacrifice of their bodies (Romans 12:1). Although they are already God's possession.
You are not your own, for you were bought with a price. So glorify God in your body. (1 Corinthians 6:19-20)
There is freedom here though (1 Corinthians 7:22-23).