The Bible often uses the word fruit not to refer to produce, but as a metaphor for virtue and good works. How does one have the power to bear fruit? It is not by self-effort. It is not by trying really hard in your own power. It is by being united to Jesus Christ through faith alone. Martin Luther explains this well. He writes concerning John 15:4,
When I am baptized or converted by the gospel, the Holy Spirit is present. The Spirit takes me as a piece of clay and make of me a new creature, which is endowed with a different mind, heart, and thoughts, that is, with true knowledge of God and a sincere trust in his grace. The very essence of my heart is changed. This makes me a new plant, one that is grafted on Christ the vine and grows from him. My holiness, righteousness, and purity do not stem from me, nor do they depend on me. They come solely from Christ and are based only in him, in whom I am rooted by faith, just as the sap flows from the stalk into the branches. Now I am like him and of his kind. Both he and I are of one nature and essence, and I bear fruit in him and through him. This fruit is not mine; it is the vine’s.
That Christ and the Christian become one loaf and one body, so that the Christian can bear good fruit—not Adam’s or his own, but Christ’s.