What is fruit? Actually the question ought to be phrased in the plural: What are fruits which a Christian can bear? The N.T. gives several answers to the question.
ONE, a developing Christian character is fruit. If the goal of the Christian life may be stated as Christlikeness, then surely every trait developed in us that reflects His character must be fruit that is very pleasing to Him. Paul describes the fruit of the Spirit in nine terms in Galatians 5:22-23, and Peter urges the development of seven accompaniments to faith in order that we might be fruitful (2 Peter 1:5-8). Two of these terms are common to both lists: love and self-control. The others are joy, peace, long-suffering, kindness, goodness, faithfulness, meekness, virtue, knowledge, endurance, piety, and brotherly love. To show these character traits is to bear fruit in one's life.
TWO, right character will result in right conduct, and as we live a life of good works we produce fruit (Colossians 1:10). This goes hand in hand with increasing in the knowledge of God, for as we learn what pleases Him, our fruitful works become more and more conformed to that knowledge. When Paul expressed how torn he was between the two possibilities of either dying and being with Christ or living on in this life, he said that living on would mean fruitful labor or work (Philippians 1:22). This phrase could mean that (1) his work itself was fruit, or (2) fruit would result from his work. In either case, his life and work were fruit. So may ours be.
THREE, those who come to Christ through our witness are fruit. Paul longed to go to Rome to have some fruit from his ministry there (Romans 1:13), and he characterized the conversion of the household of Stephanas as the first fruits of Achaia (I Corinthians 16:15).
FOUR, we may also bear fruit with our lips by giving praise to God and thankfully confessing His name (Hebrews 13:15). In other words, our lips bear fruit when we offer thankful acknowledgement to the name of God. And this is something we should do continually.
FIVE, we bear fruit when we give money. Paul designated the collection of money for the poorer saints in Jerusalem as fruit (Romans 15:28). Too, when he thanked the Philippians for their financial support of his ministry, he said that their act of giving brought fruit to their account (Philippians 4:17, KJV).