The great missionary to China was Hudson Taylor. When one thinks of the Jesus' illustration of the vine and the branches in John 15:1-8, one can see that apart from Christ, we can really do nothing for God. A branch is just a branch unless it bears fruit or two, if that branch happens to abide with the vine. The whole message of Jesus can be seen as to how there are the branches (Christians), the useless branches (unsaved people) and He is the vine.
As Christians, there is really no need to worry as Philippians 4:6 tells them not to worry but to trust God. The Christian is presented as that branch fit enough for the vine but they are not perfect branches. As Jesus says that His Father is the husbandman of the vineyard, He works on them so they bear more fruit. When the branches have some obstructions, the Christian can expect for the loving chastisement of the Father (Hebrews 12:5-7). The Christian as the useful branch is being purged so he or she can bear more fruit.
Jesus reminds Christians to abide in Him because in the flesh, the Christian is powerless. This flesh is not yet redeemed. Romans 7:13-25 has Paul complaining about his sin and wanting to be free from it. God understands but for some reason, He chooses not to make Christians instantly perfect but instead, He allows them to grow in grace first before they can be fully glorified. A mark of a true believer is one's abiding in Christ because they know the truth that without Christ, I AM NOTHING and as it goes, "Take away Christ from Christian, you get IAN or I am nothing."
To understand this union with Christ, this is the uniqueness of the Christian experience results from the union of Christ and the believer. Both the Christian and Christ are in each other in harmony where they were together. Galatians 2:20 has Christians crucified with Christ, Colossians 2:20 has Christians dead with Christ. Romans 6:4 has Christians buried with Christ. Ephesians 2:1-5 and Colossians 2:13 has Christians quickened or made alive with Christ. Colossians 3:1 and Ephesians 2:6 has Christians raised and seated in Christ. Romans 8:17 has Christians glorified with Christ. While the crucifixion, death, burial and resurrection are not in the literal sense but in a way, Christians have that union with Christ. In this way, the mark of true conversion is in the changed life of the believer from a life to sin to a live of service for the Lord Jesus Christ.
There is also a warning for false professors. Many claim to be in Christ but they are not in Christ. Such people supposedly got converted and they left. They did not lose their salvation as they were never part of Christ's family as 1 John 2:19 describes the deserters as false believers. In Matthew 7;23, Jesus tells the damned that He never knew them aside from calling them as doers of lawlessness. When that branch is useless, it's thrown into the fire and that's symbolic to how the false convert is thrown out. We all start as useless branches and it takes a real rebirth to make a useless branch into a useful branch, a goat into a sheep and a tare into a wheat.