“Either make the tree good and its fruit good, or else make the tree bad and its fruit bad; for a tree is known by its fruit.”
Here, Christ is making it clear that one either behaves in positive ways, producing positive effects, or in negative ways, producing negative effects. Often, we may be used to thinking of some things as neutral. Food, for instance, is good or neutral, in the sense that it can be used to sustain oneself or bad in that one can overeat and eat the wrong things and abuse oneself. The “use” is what Jesus has in mind here. Our moral decisions and actions are not morally neutral. Perhaps it is neutral to decide whether to eat Wheaties or Total, but that is not what Jesus means. He is speaking of decisions that have moral import–whether to be polite in a given moment, whether to love someone or remain angry with them. Those sorts of things.
These are things that affect our whole life over the time of our lives. We are call to acquire virtues and to exemplify virtues within our lives. Those are the fruits we should produce, with “thanksgiving” (eucharist) being at the heart of those virtues. Yet, if we do not do that, we cultivate vices. That is what Jesus is pointing out.
May we live a virtuous life, looking to the exemplars (saints) who have gone before us!