Present a tenth of that tithe as the Lord's offering

Numbers 18:20-32
The Lord said to Aaron, "You will have no inheritance in their land, nor will you have any share among them; I am your share and your inheritance among the Israelites. I give to the Levites all the tithes in Israel as their inheritance in return for the work they do while serving at the tent of meeting.
The Lord said to Moses, "Speak to the Levites and say to them: When you receive from the Israelites the tithe I give you as your inheritance, you must present a tenth of that tithe as the Lord's offering. You must present as the Lord's portion the best and holiest part of everything given to you.
By presenting the best part of it you will not be guilty in this matter; then you will not defile the holy offerings of the Israelites, and you will not die."
The Levites are given the nation's tithes (the tenth of all flicks herds and produce, given to God). They in turn give one-tenth to the priests, as their gift to God. What they give must be the best.
Why?
Everyone give their tithe to the Lord, and in turn Levites receive this offering and give the tenth of all they got to the Lord. It is express the heart to observe the Lord and honor the name of the Lord.
Aaron and his tribes  priests for they were serving the Lord so the Lord asked them have no inheritance in their land and business. For if they involve in the world, they will lost their mission and holy identity.
How?
To be a Christian have to live Jesus, far away from dirty thing and the darkness in the world. And obey the command to give tithe to the Lord.