One question that has been raised by recent sermons is this: Because living faith will necessarily produce faithfulness (good works) in those who are united to Christ, how much "fruit" is necessary to prove the presence of living faith? John 15 seems to suggest that this is the wrong sort of question to ask, because Jesus doesn't give us a quota for fruitfulness. He calls us to abide in Him and obey His commandments, trusting Him to make us fruitful. We have an active part to play in the lifelong process of sanctification, but the amount of fruit we bear is ultimately up to God. If we're worried that we won't have enough fruit to make God happy, then we've missed the whole point. We can only bear fruit if we've already been made alive in Christ Jesus and filled with His Spirit. Our assurance of salvation must never rest in ourselves or in our good works, because even our good works are God's gifts. Keeping looking to Jesus, and keep trusting His promises by soaking up God's word and availing yourself of all the means of grace God has provided for our nourishment.
We should be very careful about performing "fruit inspections" on ourselves or others (Gal. 5:25-6:1), but the important thing is to look at long-term patterns and trajectories (1 John 3:4-10). How do you tell whether a tree in your yard is alive or dead? When you look at a fruit tree, you can probably tell whether it is dead or alive regardless of how much fruit it has on it at the moment. You can probably tell whether it is diseased or healthy, even though it may not bear much fruit in a given season. Jesus promises to prune branches that are alive so that they will become more fruitful, and that pruning process may lead to a brief decrease of fruit that is followed by a great increase. According to Psalm 1:3-4, the blessed man "is like a tree planted by streams of water that yields its fruit in its season, and its leaf does not wither."