Matthew makes it sound simple
A good tree cannot bear bad fruit, nor can a rotten tree bear good fruit. Every tree that does not bear good fruit will be cut down and thrown into the fire. So by their fruits you will know them.
Every metaphor is limited and the limit of this one is that we are not trees. We are human beings, rational beings. Once baptized we are the sons and daughters of God, members of the body of Christ. But the fruit we bear can and usually is of varying quality, not all bad or all good.
A tree is a passive recipient. It received its genetic makeup, it receives water and nutrients. These things determine the kind of fruit it will bear. We are anything but passive. Unlike the tree that has no choice in the matter, we do. We decide on a day to day and minute to minute basis what kind of fruit we will bear.
Do we have genetic predispositions? Yes. Are we the products of our upbringing? Yes. But are we determined by them? Christianity's response is a categorical NO. We are always to some degree free, and to that degree we are responsible.
Today and every day we are given a new opportunity to choose the kind of fruit that we will bear.
