My older sister wrote and asked me if I considered tithing a New Testament practice. I just re-read my answer, and I think it's worth sharing.
My dear dearest older sister -
You're trying to get me in even deeper trouble with everybody we know, aren't you?
Oh well - we're all grownups. So here goes. About the tithe.
Whatever else it may be, it's certainly not a New Testament practice - or more specifically, it is not a Christian practice in the New Testament. Jesus speaks of it only once, when he criticizes people for paying so much attention to tithing when they should pay more attention to issues of mercy and justice. And the people he spoke to were not believers, they were not His disciples.
After the resurrection, during the infancy of the ecclesia, the only other mention of tithing is in Hebrews 7 - that whole passage about Melchizidek. And if you read that passage in context, you'll see that even then, the author is not making a point about tithing, he's making a point about the supremacy of Jesus Christ. And in making that point, at no point does he say that we should tithe.
Neither do any other N.T. authors. Paul, Peter, James, John - in all of their instructions to believers, none of them ever says squat about tithing. As a doctrinal subject, it simply ISN'T THERE.
Not only is there no teaching about tithing; there are no examples of tithing - not one church, not one believer, is ever described as doing it. If "practice" is what people do, then tithing is no more a New Testament practice than is playing golf.
On the other hand, GIVING - without a mathematical formula attached - is very much in evidence in the N.T. : in Jesus' teaching to his disciples (Luke 6.38), in Paul's teaching (2 Corinthians 9), and in what believers actually did (Acts 2.42-47, 4.32-37; 2 Corinthians 8-9; Philippians 4:15-20).
The N.T. offers plenty of reasons to give; I think the #1 reason is this: the God who lives in us is generous. If we paid more attention to His life within us, we'd be too busy giving to think about tithing.