The purpose of the tithe was twofold. First of all it was to teach the Israelites to put God first in their lives (Deut 14:23). The Israelites in the Old Testament were a spiritually dead people. They were not born again. They did not have God’s Spirit living inside of them. Today if you are born again then Jesus has become the Lord of your life, his spirit is within you and you have already put God first in your life. At some point in your life you realized that with you being the lord of your own life, your life was going nowhere and you made Jesus Lord of your life. Now He wants to direct your giving on a voluntary, agreed upon basis rather than the debtor/creditor relationship that is the tithe.
Secondly the tithe was a tax. The nation of Israel was a theocracy (i.e. God ruled) with God being the head of state. His directives came through the prophets, judges and the law. The tithe/tax was necessary so that the government/church of Israel would have the resources necessary for the operation of a sovereign state. In Israel the church was the government and was responsible for carrying on the functions that most governments of today are responsible for, things such as caring for the poor, national defence and maintenance of infrastructure etc (Deut 26:12). Today we don’t live in a theocracy but in a democracy which means “the rule of the people”. God doesn’t rule as the head of state anymore but as Lord of those individuals who accept Him as Lord. Those individuals then elect the head of state.
The word tithe is an ancient transliterated word and was never translated into English. Translated it means “a tenth part” and was common as a tax all throughout the ancient world. In Israel the tithe was only on crops, fruit or animals grown in Israel (Lev 27:30&32). Israel was an agrarian society so many people worked in agriculture and those who worked for wages were not required to pay tithes on their income. The tithe was only on crops grown and cattle raised in Israel, on that which came from the land because God had given them the land to start with. The tithe was only on that which God increased through the miracle of growth on land He had provided, not on the self efforts of man providing for himself. Those who worked for wages were not required to tithe on their wages. There is no record of Joseph or Jesus paying tithes on their income as carpenters, for that matter there is no record of Peter paying tithes as a fisherman or Paul as a tentmaker.
The tithe was based on a debtor/creditor relationship between the Israelites and God. New Testament giving is voluntary and Spirit led, based on a love relationship between you and your Father. With these thoughts in mind then for a man to tithe on that which he increases through own self effort is, in effect, putting himself first in his life and making himself his own lord. For a church to demand that people tithe on their income not only contradicts the purpose of the tithe but it is a double tax, taxing people for services it no longer provides. Services that they have already paid for and their governments are already providing.