Two kinds of giving are taught consistently throughout Scripture: giving to the government (always compulsory), and giving to God (always voluntary).
The issue has been greatly confused, however, by some who misunderstand the nature of the Old Testament tithes. Tithes were not primarily gifts to God, but taxes for funding the national budget in Israel.
Because Israel was a theocracy, the Levitical priests acted as the civil government. So the Levite's tithe (Leviticus 27:30-33) was a precursor to today's income tax, as was a second annual tithe required by God to fund a national festival (Deuteronomy 14:22-29). Smaller taxes were also imposed on the people by the law (Leviticus 19:9-10; Exodus 23:10-11). So the total giving required of the Israelites was not 10 percent, but well over 20 percent. All that money was used to operate the nation.
All giving apart from that required to run the government was purely voluntary (cf. Exodus 25:2; 1 Chronicles 29:9). Each person gave whatever was in his heart to give; no percentage or amount was specified.
New Testament believers are never commanded to tithe. Matthew 22:15-22 and Romans 13:1-7 tell us about the only required giving in the church age, which is the paying of taxes to the government. Interestingly enough, we in America presently pay between 20 and 30 percent of our income to the government--a figure very similar to the requirement under the theocracy of Israel.
The guideline for our giving to God and His work is found in 2 Corinthians 9:6-7: "Now this I say, he who sows sparingly shall also reap sparingly; and he who sows bountifully shall also reap bountifully. Let each one do just as he has purposed in his heart; not grudgingly or under compulsion; for God loves a cheerful giver."
For further study:
John MacArthur, God's Plan for Giving (tape series).