"I am the true vine, and My Father is the vinedresser. Every branch in Me that does not bear fruit He takes away; and every branch that bears fruit He prunes, that it may bear more fruit" (John 15:1,2).
The Old Testament described the nation of Israel as a vine or a vineyard (cf. Psa. 80:8-16; Isa. 5:1-7; Jer. 2:21; Hosea 10:1). They were brought out of Egypt and planted in Canaan, after its inhabitants were cut down. God blessed them richly and provided everything they needed to grow large and strong, but they forsook Him and were "burned with fire" and "cut down" for their disobedience (Psa. 80:16). They were expected to bear good fruit, but instead they brought forth "wild grapes" (Isa. 5:4).
Jesus is the true vine in the sense that He is genuine and faithful (unlike Israel was). He is able to supply the branches (i.e., His disciples, John 15:5) with the spiritual nourishment needed. The heavenly Father is the one who attends to the vineyard. He is not some hired laborer but the owner, and He will definitely give the true vine and its branches His closest attention and utmost care.
Let it be observed that the fact that Jesus is the true vine implies that there are, or could be, false vines (e.g., those who offer spiritual nourishment but cannot deliver such)!
Like any good vinedresser, the Father will cut off the branches that do not bear fruit, and He will prune those branches that are bearing fruit that they may become even more productive.
This verse clearly teaches the possibility of apostasy. One who is "in" Christ will be severed from Him if he fails to bear fruit as the Father expects. Those who are fruit-bearing branches of the Lord are pruned (or cleansed) through teaching, training, trials, and discipline in order that their capacity for bearing fruit will increase.
Some mistakenly believe that the only fruit of a Christian is the conversion of other disciples. Although helping to convert a lost soul is certainly a part of fruit-bearing, there is much more involved, specifically the development of certain important personal characteristics (cf. Gal. 5:22,23).
It is important to understand that the branches Jesus speaks of here are individual disciples of the Lord (cf. John 15:5); the branches are not denominations, as some have suggested in an attempt to justify the concept of denominationalism. Such did not even exist until many years after Jesus spoke these words. Also, from a purely logical standpoint, a vine that brings forth pumpkins on one branch, for example, will not bring forth watermelon, cucumbers, etc., upon the others! Jesus is speaking to His followers as individuals in this context, and they derive spiritual life from Him, the true vine to which they are attached as branches.
Paul teaches a related idea when he speaks of the church as the body of Christ and His disciples as individual members of that one body (cf. I Cor. 12:12-27; Eph. 1:22,23). No part of the body, if it is severed from its source of life, can function properly or live (cf. Col. 2:19). Salvation is "in" Christ, never apart from Him (cf. II Tim. 2:10; Gal. 3:27)!