Is tithing a sin?

I came across an interesting article a while ago on a Jewish website saying that tithing was a sin. At first I thought that it was ludicrous, but after reading the article I realized they had a point. I couldn't find the exact article, but I found one that was very similar.

Here are seven interesting points taken from http://bible-truths.com/tithing.html;;
1.) Abraham never tithed from his own personal property or livestock
2.) Jacob wouldn't tithe until God blessed him first
3.) Only Levite priests could collect tithes; there are no levite priests anymore.
4.) Only food products from the land were tithable
5.) Money was never a tithable commodity
6.) Christian converts were never asked to tithe anything to the Church
7.) Tithing first appears centuries after completion of the Bible

There are many interesting points in this article including the following;

1.) Only landowners tithed
2.) Only Products of the land were tithed
3.) Only Levites could receive the tithe
4.) Tithing was a law of Moses
5.) Christians are not under the law of Moses.

According to the author of the article, most pastors know that tithing is no longer required but do not want to tell the congregation this due to the fear of church bills going unpaid. The author also lists that giving is a matter of the heart and lists verses that support giving to the church in gifts of love, not in a matter of mandatory tithing.

What do you think about this?