I first became aware of tithing as a teenager when my pastor preached on the importance of supporting the local congregation. His sermon drew on Galatians 6:6: “Anyone who receives instruction in the word must share all good things with his instructor.”

Freewill offerings were commonplace in the Old Testament era. The three tithes were never the ceiling of giving, merely the floor. 

When I read the Bible, it appears that tithing is the biblical minimum, the starting place, the floor of giving, not its ceiling. Tithes were to keep from stealing from God. Offerings were the beginning of giving.