The First Tithe (Old Covenant)
30“‘All the tithe of the land, whether of the seed of the land or of the fruit of the trees, is Yahweh’s. It is holy to Yahweh. Leviticus 27:30
19All the wave offerings of the holy things, which the children of Israel offer to Yahweh, I have given you, and your sons and your daughters with you, as a portion forever. It is a covenant of salt forever before Yahweh to you and to your offspring with you.” 20Yahweh said to Aaron, “You shall have no inheritance in their land, neither shall you have any portion among them. I am your portion and your inheritance among the children of Israel. 21“To the children of Levi, behold, I have given all the tithe in Israel for an inheritance, in return for their service which they serve, even the service of the Tent of Meeting. Numbers 18:19-21
Yahweh first made provision for His ministers, the Levites. These servants of the Old Covenant lived off of the first tithe (tenth) which were given daily by all those in covenant with Yahweh. This was a commandment from God Himself; to disobey was to break the commandment of the Law and incur all the curses contained therein. The first tithe was such a part of Yahweh's mind that He called them "His." Thus, in reality, the first tithe of each Israelite's increase did not even belong to them. It was God's from the beginning.
As said above, the tithe is already Yahweh's, therefore to not give it back to Him is to rob Him.
8Will a man rob God? Yet you rob me! But you say, ‘How have we robbed you?’ In tithes and offerings. 9You are cursed with the curse; for you rob me, even this whole nation. Malachi 3:8-9
The Lord Jesus showed us that it is right and good to tithe on all of our increase to the priests, just as the Pharisees had practiced, even though the other weightier matters of the Law they had left undone. Thus, the Christ upheld the commandment of the Law with respect to tithing, as He did with the rest of God's Law (Matthew 5:17-18).
23“Woe to you, scribes and Pharisees, hypocrites! For you tithe mint, dill, and cumin, and have left undone the weightier matters of the law: justice, mercy, and faith. But you ought to have done these, and not to have left the other undone. Matthew 23:23
See also (Luke 11:42).
Christ therefore reminds us that to consciously disobey God, even in one of the least commandments, is sin and is cause for a lowering of position in the coming Kingdom (Matthew 5:19). We should obey all His commandments, out of love (1 John 5:3).
We see that the reason for this first tithe is so that there would be food in God's House (Malachi 3:10). The Levitical priesthood had no other inheritance except the tithes of the children of Israel. If Israel did not bring in their first tithe every day, God's Temple remained desolate. Because daily sacrifices were required, God had set up His nation so that there would always be food for His servants by means of those they ministered to at the altar. Thus, we see the importance of giving the first tithe under the Law, and should not be surprised that Yahweh uses such strong language when referring to it.