To "tithe" means to give a tenth of "all the increase" (Deuteronomy 14:22 Deuteronomy 14:22You shall truly tithe all the increase of your seed, that the field brings forth year by year.
American King James Version×) derived from one's produce, property or income, for the support of a religious purpose. The motivation to tithe is a worshipful recognition of God as the Creator and Possessor of everything, including ourselves.
Although tithing became a written law under the covenant God made with Israel, it was practiced among those who were faithful to God before that covenant. Abraham, after his defeat of the four kings, tithed on the spoils of the war to Melchizedek, priest of God Most High (Genesis 14:18-22 Genesis 14:18-22  And Melchizedek king of Salem brought forth bread and wine: and he was the priest of the most high God.
 And he blessed him, and said, Blessed be Abram of the most high God, possessor of heaven and earth:
 And blessed be the most high God, which has delivered your enemies into your hand. And he gave him tithes of all.
 And the king of Sodom said to Abram, Give me the persons, and take the goods to yourself.
 And Abram said to the king of Sodom, I have lift up my hand to the LORD, the most high God, the possessor of heaven and earth,
American King James Version×). Abraham obviously understood giving a tenth as the appropriate way to honor God with one's physical possessions. Note that Abraham gave the tenth to Melchizedek, a representative of the Creator.
Abraham recognized the underlying premise for giving a tithe to God: He is the actual "Possessor of heaven and earth" who made his victory, and all blessings, possible. God reminds us throughout the Bible, and people of God respectfully acknowledge, that everything belongs to God (Exodus 19:5 Exodus 19:5Now therefore, if you will obey my voice indeed, and keep my covenant, then you shall be a peculiar treasure to me above all people: for all the earth is mine:
American King James Version×; Job 41:11 Job 41:11Who has prevented me, that I should repay him? whatever is under the whole heaven is mine.
American King James Version×; Psalms 24:1 Psalms 24:1The earth is the LORD's, and the fullness thereof; the world, and they that dwell therein.
American King James Version×; 50:12; Haggai 2:8 Haggai 2:8The silver is mine, and the gold is mine, said the LORD of hosts.
American King James Version×). "And you shall remember the LORD your God," Moses told Israel, "for it is He who gives you power to get wealth" (Deuteronomy 8:18 Deuteronomy 8:18But you shall remember the LORD your God: for it is he that gives you power to get wealth, that he may establish his covenant which he swore to your fathers, as it is this day.
American King James Version×). Tithing is thus, first and foremost, an act of worshipful recognition of God as our source of existence, blessing and providence.
Jacob also followed the example of his grandfather Abraham. When God reconfirmed to him the promises He had made to Abraham, Jacob promised God, "Of all that You give me I will surely give a tenth [tithe] to You" (Genesis 28:20-22 Genesis 28:20-22  And Jacob vowed a vow, saying, If God will be with me, and will keep me in this way that I go, and will give me bread to eat, and raiment to put on,
 So that I come again to my father's house in peace; then shall the LORD be my God:
 And this stone, which I have set for a pillar, shall be God's house: and of all that you shall give me I will surely give the tenth to you.
American King James Version×).
The practice of tithing was later incorporated into the covenant with Israel as a written law. The tribe of Levi, which was not given an inheritance of land from which they could derive increase (Numbers 18:23 Numbers 18:23But the Levites shall do the service of the tabernacle of the congregation, and they shall bear their iniquity: it shall be a statute for ever throughout your generations, that among the children of Israel they have no inheritance.
American King James Version×), was to receive the tithe of the agricultural produce in return for their service to the nation. The Levites, based on what they received in tithes from the people, in turn tithed to the priests (18:26-28).
As time passed, the tithe was carelessly neglected in postexilic Judah, for which God corrected the nation in the strongest of terms (Malachi 3:8-10 Malachi 3:8-10  Will a man rob God? Yet you have robbed me. But you say, Wherein have we robbed you? In tithes and offerings.
 You are cursed with a curse: for you have robbed me, even this whole nation.
 Bring you all the tithes into the storehouse, that there may be meat in my house, and prove me now herewith, said the LORD of hosts, if I will not open you the windows of heaven, and pour you out a blessing, that there shall not be room enough to receive it.
American King James Version×). Failure to tithe, God said, was robbing Him, and the people were consequently cursed. Yet He also promised that renewed obedience in tithing would result in blessings from Him so abundant that "there [would] not be room enough to receive it."
Some centuries later, Jesus Himself clearly upheld the practice of tithing. "Woe to you, scribes and Pharisees, hypocrites! For you pay tithe of mint and anise and cummin, and have neglected the weightier matters of the law: justice and mercy and faith. These you ought to have done, without leaving the others undone" (Matthew 23:23 Matthew 23:23Woe to you, scribes and Pharisees, hypocrites! for you pay tithe of mint and anise and cummin, and have omitted the weightier matters of the law, judgment, mercy, and faith: these ought you to have done, and not to leave the other undone.
American King James Version×). Christ plainly confirmed His will that tithing should indeed be practiced, along with sincere adherence to other "weightier [spiritual] matters."
As tithes and offerings in Israel were given to the tribe of Levi for their livelihood and service to God, the Church in the New Testament provided financial support for the ministry to carry on their work. Instances of, and principles relating to, this practice are found in Luke 10:1 Luke 10:1After these things the LORD appointed other seventy also, and sent them two and two before his face into every city and place, where he himself would come.
American King James Version×, 7-8; 1 Corinthians 9:7-14 1 Corinthians 9:7-14  Who goes a warfare any time at his own charges? who plants a vineyard, and eats not of the fruit thereof? or who feeds a flock, and eats not of the milk of the flock?  Say I these things as a man? or said not the law the same also?  For it is written in the law of Moses, You shall not muzzle the mouth of the ox that treads out the corn. Does God take care for oxen?  Or said he it altogether for our sakes? For our sakes, no doubt, this is written: that he that plows should plow in hope; and that he that threshes in hope should be partaker of his hope.  If we have sown to you spiritual things, is it a great thing if we shall reap your carnal things?  If others be partakers of this power over you, are not we rather? Nevertheless we have not used this power; but suffer all things, lest we should hinder the gospel of Christ.  Do you not know that they which minister about holy things live of the things of the temple? and they which wait at the altar are partakers with the altar?  Even so has the Lord ordained that they which preach the gospel should live of the gospel.
American King James Version×; 2 Corinthians 11:7-9 2 Corinthians 11:7-9  Have I committed an offense in abasing myself that you might be exalted, because I have preached to you the gospel of God freely?  I robbed other churches, taking wages of them, to do you service.  And when I was present with you, and wanted, I was chargeable to no man: for that which was lacking to me the brothers which came from Macedonia supplied: and in all things I have kept myself from being burdensome to you, and so will I keep myself.
American King James Version×; Philippians 4:14-18 Philippians 4:14-18  Notwithstanding you have well done, that you did communicate with my affliction.  Now you Philippians know also, that in the beginning of the gospel, when I departed from Macedonia, no church communicated with me as concerning giving and receiving, but you only.  For even in Thessalonica you sent once and again to my necessity.  Not because I desire a gift: but I desire fruit that may abound to your account.  But I have all, and abound: I am full, having received of Epaphroditus the things which were sent from you, an odor of a sweet smell, a sacrifice acceptable, well pleasing to God.
American King James Version×and Hebrews 7.
Today the United Church of God continues the teaching that tithing is a universal law and that one's willing obedience to this law reflects the unselfish, giving nature of our Creator and Provider.
Through tithing that springs from willing and cheerful giving (2 Corinthians 9:6-8 2 Corinthians 9:6-8  But this I say, He which sows sparingly shall reap also sparingly; and he which sows bountifully shall reap also bountifully.
 Every man according as he purposes in his heart, so let him give; not grudgingly, or of necessity: for God loves a cheerful giver.
 And God is able to make all grace abound toward you; that you, always having all sufficiency in all things, may abound to every good work:
American King James Version×), God has provided the perfect financial system to take care of the needs of His work (Matthew 24:14 Matthew 24:14And this gospel of the kingdom shall be preached in all the world for a witness to all nations; and then shall the end come.
American King James Version×; 28:19-20), the personal need to attend His festivals and the need to care for the poor (Deuteronomy 14:22-27 Deuteronomy 14:22-27  You shall truly tithe all the increase of your seed, that the field brings forth year by year.  And you shall eat before the LORD your God, in the place which he shall choose to place his name there, the tithe of your corn, of your wine, and of your oil, and the firstborn of your herds and of your flocks; that you may learn to fear the LORD your God always.  And if the way be too long for you, so that you are not able to carry it; or if the place be too far from you, which the LORD your God shall choose to set his name there, when the LORD your God has blessed you:  Then shall you turn it into money, and bind up the money in your hand, and shall go to the place which the LORD your God shall choose:  And you shall bestow that money for whatever your soul lusts after, for oxen, or for sheep, or for wine, or for strong drink, or for whatever your soul desires: and you shall eat there before the LORD your God, and you shall rejoice, you, and your household,  And the Levite that is within your gates; you shall not forsake him; for he has no part nor inheritance with you.
American King James Version×, 28-29).
For more information, see What Does the Bible Teach About Tithing?