The Old Testament contains a vast amount of material about giving a portion of one’s harvest, properties, or ownings back to God. Many commandments in the Old Testament mandate giving of one’s “first fruits” i.e. the first of the fruit or grain harvest, or the first of the newborn livestock. The Hebrew word for this offering is translated as “tithe” or “a tenth-part” with the emphasis being that the first fruits are intentionally offered.
The New Testament doesn’t adopt the language of tithing or mandate that a particular percent be given for the support of a religious institution. That said, in Mark 12:41-44, Jesus praised a woman who put the equivalent of a penny into the Temple treasury (similar to the collection basket at churches today), but denounced someone else who gave a tenth of his income with a public show of self-righteous grandeur (Luke 18:9-14).
The United States Conference of Catholic Bishops suggests we give a “planned, proportionate, and sacrificial” amount, yet like the New Testament does not tell us what percentage of our income that should be. What is a clear biblical challenge is that we give of our “first fruits” rather than our “leftovers,” those one dollar bills that we sometimes slip into the collection basket on Sunday because we’ve already spent our paychecks. A more biblical approach to tithing would be to include our charitable giving as a non-negotiable line item in our monthly budgets right along with food, utilities and rent.