Do we have to tithe? Why does God need us to tithe? What are the results of NOT tithing?
The strongest words with respect to the benefits of paying tithing, and the consequences for not doing so, can be found in Malachi 3: 8-12:
8 Will a man rob God? Yet ye have robbed me. But ye say, Wherein have we robbed thee? In tithes and offerings.
9 Ye are cursed with a curse: for ye have robbed me, even this whole nation.
10 Bring ye all the tithes into the storehouse, that there may be meat in mine house, and prove me now herewith, saith 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.
11 And I will rebuke the devourer for your sakes, and he shall not destroy the fruits of your ground; neither shall your vine cast her fruit before the time in the field, saith the Lord of hosts.
12 And all nations shall call you blessed: for ye shall be a delightsome land, saith the Lord of hosts.
However, it appears that in the Christian period, just as with many other provisions of the Law, tithing was superseded by a higher law. Towards the end of both Acts 2 and Acts 4 it explains that the Christian community had all things in common, consecrating not a tithe (tenth) of their belongings but instead everything they owned to the general welfare of the church in order to do away with poverty. (It's worth noting that, while there are many obvious differences and incompatibilities between Christianity and communism, Karl Marx's famous "from each according to his ability, to each according to his need" was inspired by this part of Acts.)
The location of the chapter break between Acts 4 and Acts 5 is unfortunate, as Acts 5 begins with a direct continuation of the discussion of the treatment of the consecration of property that Acts 4 ended with. Here we see the story of a man and his wife who sold their property and gave part of the money to the Apostles, but kept back part of it to enrich themselves. Apparently the Lord takes this very seriously, (Galatians 6: 7--God is not mocked,) as He struck both of them dead when they tried to reaffirm their lie when questioned by Peter!
I'm not aware of any church today that practices this doctrine of total consecration, but the principle is still there. At the very least, a true believer should be willing to give up all that he posesses if it is needed to further the Lord's cause. (See Matthew 13: 44-46, Luke 18: 18-25, and especially Luke 18: 29-30 for the Savior's own words on this matter. See also Luke 12: 34/Matthew 6: 21--"Where your treasure is, there will your heart be also." The entire point of the Gospel is to improve our hearts and turn them towards heaven instead of the things of this world.)
I take 'why do we have to tithe' a step further:
24 “No one can serve two masters. Either you will hate the one and love the other, or you will be devoted to the one and despise the other. You cannot serve both God and money.
By requiring the tithe, God is teaching us to put our trust in Him over money. So HAVE TO should be WANT TO.
I know this question has an accepted answer but it seems that one scripture has been missed:
Matt 23:23 NKJV “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.
In the words of Jesus "These you ought to have done, without leaving the others undone." In the NIV translation it says
You should have practiced the latter, without neglecting the former. So from that we can learn that Jesus was telling us to do both tithing and justice/mercy/faith.
One argument for not tithing is that it is a jewish law and we are not bound by it any more. While it is true we are not bound by the law tithing came before and was made into law.
Hebrews 7:2a NIV and Abraham gave him a tenth of everything ... *snip*
(also read Genesis 14:20)
The ten commandments and law were not around in the time of Abraham and therefore it was a principle known to Abraham (I assume from God) to give one tenth, a tithe, to the priest which in this case was Melchizedek.