Q: I understand from your teaching that tithing was part of the Mosaic law and that we are no longer under the Law. I also know that God is pleased with a cheerful giver. But recently someone reminded me that Abraham tithed in the book of Genesis in an act that predates the Law. Also, Jesus speaks of tithing in Mathew 23:23 as something that is important. Now I'm confused. Should we tithe or not?
You're right, tithing does predate the Mosaic Law, but just because someone is seen doing something in an Old Testament narrative, that doesn't mean we are commanded to do it today. But more than that, it is important to realize that the New Covenant is not about keeping rules. God wants His children to be Spirit-led — not governed by external regulations. That's why the Apostle Paul wrote:
Each one must give as he has decided in his heart, not reluctantly or under compulsion...2 Corinthians 9:7 (ESV)
What Paul is describing in 1 Corinthians is precisely the kind of giving you're seeing in Genesis 14. Abraham didn't give a tenth of his possessions to Melchizedek because he had to or because he was under some external compulsion to comply. He gave because he had determined in his heart to give. That's the kind of giving we are exhorted to imitate under the New Covenant.
In the Matthew passage Jesus is simply acknowledging the principle of tithing in the Scripture, but once again, His comments are given in the hearing of Jews prior to the establishing of the New Covenant. The Apostle Paul's remarks in 2 Corinthians are God's present-day instructions for the Church on the subject of Spirit-led giving.