D&C 119

As the Lord answered the Prophet Joseph Smith's supplication -

"O Lord, show unto thy servants how much thou requirest of the properties of thy people for a tithing."  (Doctrine and Covenants | Section 119:Heading)

...of which the Lord answered -

"...those who have thus been tithed shall pay one-tenth of all their interest annually; and this shall be a standing law unto them forever, for my holy priesthood, saith the Lord."  (Doctrine and Covenants | Section 119:4)

At the time of this revelation, the word "interest" suggested income.  To which this law of tithing -

"...shall be an ensample unto all the stakes of Zion..."  (Doctrine and Covenants | Section 119:7)

Which in my mind, means all members of the Church.  The Lord then provides this warning for those that fail to live this law -

"...if my people observe not this law, to keep it holy, and by this law sanctify the land of Zion unto me, that my statutes and my judgments may be kept thereon, that it may be most holy, behold, verily I say unto you, it shall not be a land of Zion unto you."  (Doctrine and Covenants | Section 119:6)

I have blogged much on the meaning of Zion.  In my mind, Zion is living celestial law on earth or in other words, living in a manner to someday be worthy to live with God.  If we cannot live this law, we will not have the ability to live with God or receive the blessing of a Zion life.

Some have wondered what one-tenth of income means.  Elder John A. Widstoe provided some wise guidelines -

“Tithing means one-tenth. Those who give less do not really pay tithing; they are lesser contributors to the Latter-day cause of the Lord. Tithing means one-tenth of a person's income, interest, or increase. The merchant should pay tithing upon the net income of his business, the farmer upon the net income of his farming operations; the wage earner or salaried man upon the wage or salary earned by him. Out of the remaining nine-tenths he pays his current expenses, taxes, savings, etc. To deduct living costs, taxes, and similar expenses from the income and pay tithing upon the remainder does not conform to the Lord's commandment. Under such a system most people would show nothing on which to pay tithing. There is really no place for quibbling on this point. Tithing should be given upon the basis of our full earned income. If the nature of a business requires special interpretation, the tithe payer should consult the father of his ward, the bishop” (Evidences and Reconciliations, page 86).

I will also add a personal opinion.  I believe the reason that the Lord did introduce the Law of Tithing we because I believe the United Order would never work with general commerce with the rest of the world.  Obviously as the Church would grow, even globally in a global economy, this is the only way the law of consecration could be exercised.