Read John 15.
I do not know the origin of the following statement but it is classic. "Spiritual fruit is the life of Jesus Christ reproduced in us by the Holy Spirit and reproduced in others as we give away our faith."
In this chapter, Jesus compared the Christian life to branch on a grape vine. With God overseeing His vineyard and Christ serving as the vine in the analogy, the branch is expected to produce fruit. It is the natural result of life and health in the branch derived from the vine. No fruit reveals no health and/or no life in the branch. For the overseer of the vineyard, the solution for lack of health in such cases is pruning. In the case of lack of life, the branch is taken away to the fire.
At a particular time of the year, leaves sprout on a vineyard producing a beautiful and attractive sight for photography. But, it will not be long before the workers will cut away all those leaves. When asked why he was ruining such beautiful sight, one overseer quickly replied, "We are not in the leaf business." If not removed, those leaves would sap life from the branch instead of producing fruit.
Jesus said, "So, every healthy tree bears good fruit...Every tree that does not bear good fruit is cut down...Thus you will recognize them by their fruits." (Matthew 7:17-20)
False teachers are called "...fruitless trees..." (Jude 12)
The New Testament uses the word "fruit" in both ways--the result of what God does in us and the result of what God does through us.
When the Holy Spirit is in control of a follower of Jesus, there is fruit. This is in direct contrast to "the works of the flesh," or when a person tries to run their own lives.
The works (the product or fruit) of the flesh are evident and include: "...sexual immorality, impurity, sensuality, idolatry, sorcery, enmity, strife, jealousy, fits of anger, rivalries, dissensions, divisions, envy, drunkenness, orgies, and things like these. I warn you, as I warned you before, that those who do such things will not inherit the kingdom of God." (Galatians 5:19-21) These words compare with Jesus' warning in John 15:6.
The fruit (singular) of the Holy Spirit includes: "...love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control..." (Galatians 5:22-23)
"By this is my Father glorified, that you bear much fruit and so prove to be my disciples." (John 15:8)