
The common analogy of likening faith to a fruit tree is unbiblical. It makes works inherent in faith, just like fruit is inherent in the seed. The analogy teaches a subtle form of works salvation. It has been the cause of endless suffering for Christians with sensitive consciences.
If I had to guess, I assume the fruit tree analogy was taken from the parable of the sower. However, in the parable of the sower, neither the seed nor the soil refer to faith. Also, the fruits of the Christian life are not referred to as the fruits of faith, but the fruits of the Spirit, among which faith is just one of the fruits (see Galatians 5:22-23 and 1 Corinthians 12:1-10). And this accords with John 15 where it is not the act of abiding that bears fruit, but the life-giving sap that flows through the vine.