From: John 15:1-8
The Vine and the Branches
(Jesus said to His disciples,)  "I am the vine, and My Father is the vinedresser.  Every branch of mine that bears no fruit, He takes away, and every branch that does bear fruit He prunes that it may bear more fruit.  You are already made clean by the word which I have spoken to you.  Abide in Me, and I in you. As the branch cannot bear fruit by itself, unless it abides in the vine, neither can you, unless you abide in Me.  I am the vine, you are the branches. He who abides in Me, and I in him, he it is that bears much fruit, for apart from Me you can do nothing.  If a man does not abide in Me, he is cast forth as a branch and withers; and the branches are gathered, thrown into the fire and burned.  If you abide in Me, and My words abide in you, ask whatever you will, and it shall be done for you.  By this My Father is glorified, that you bear much fruit, and so prove to be My disciples."
1. This comparison of the chosen people with a vine was used in the Old Testament: Psalm 80 speaks of the uprooting of the vine in Egypt and its re-planting in another land; and in Isaiah's Song of the Vineyard (5:1-7) God complains that despite the care and love He has lavished on it, His vineyard has yielded only wild grapes. Jesus previously used this imagery in His parable about the murderous tenants (Matthew 21:33-43) to signify the Jew's rejection of the Son and the calling of the Gentiles. But here the comparison has a different, more personal meaning: Christ explains that He Himself is the true vine, because the old vine, the original chosen people, has been succeeded by the new vine, the Church, whose head is Christ (cf. 1 Corinthians 3:9). To be fruitful one must be joined to the new, true vine, Christ: it is no longer a matter of simply belonging to a community but of living the life of Christ, the life of grace, which is the nourishment which passes life on to the believer and enables him to yield fruits of eternal life. This image of the vine also helps understand the unity of the Church, Christ's mystical body, in which all the members are intimately united with the head and thereby are also united to one another (1 Corinthians 12:12-26; Romans 12:4-5; Ephesians 4:15-16).
2. Our Lord is describing two situations: that of those who, although they are still joined to the vine externally, yield no fruit; and that of those who do yield fruit but could yield still more. The Epistle of St. James carries the same message when it says that faith alone is not enough (James 2:17). Although it is true that faith is the beginning of salvation and that without faith we cannot please God, it is also true that a living faith must yield fruit in the form of deeds. "For in Christ Jesus neither circumcision nor uncircumcision is of any avail, but faith working through love" (Galatians 5:6). So, one can say that in order to produce fruit pleasing to God, it is not enough to have received Baptism and to profess the faith externally: a person has to share in Christ's life through grace and has to cooperate with Him in His work of redemption.
Jesus uses the same verb to refer to the pruning of the branches as He uses to refer to the cleanness of the disciples in the next verse: literally the translation should run: "He cleanses him who bears fruit so that he bear more fruit". In other words, He is making it quite clear that God is not content with half-hearted commitment, and therefore He purifies His own by means of contradictions and difficulties, which are a form of pruning, to produce more fruit. In this we can see an explanation of the purpose of suffering: "Have you not heard the Master Himself tell the parable of the vine and the branches? Here we can find consolation. He demands much of you for you are the branch that bears fruit. And He must prune you `ut fructum plus afferas": to make you bear more fruit'.
"Of course: that cutting, that pruning, hurts. But, afterwards, what richness in your fruits, what maturity in your actions" ([St] J. Escriva, "The Way", 701).
3. After washing Peter's feet Jesus had already said that His Apostles were clean, though not all of them (cf. John 13:10). Here, once more, He refers to that inner cleansing which results from accepting His teachings. "For Christ's word in the first place cleanses us from errors, by instructing us (cf. Titus 1:9) [...]; secondly, it purifies our hearts of earthly affections, filling them with desire for Heavenly things [...]; finally, His word purifies us with the strength of faith, for `He cleansed their hearts by faith' (Acts 15:9)" (St. Thomas Aquinas, "Commentary on St. John, in loc.").
4-5. Our Lord draws more conclusions from the image of the vine and the branches. Now He emphasizes that anyone who is separated from Him is good for nothing, like a branch separated from the vine. "You see, the branches are full of fruit, because they share in the sap that comes from the stem. Otherwise, from the tiny buds we knew just a few months back, they could not have produced the sweet ripe fruit that gladdens the eye and make the heart rejoice. Here and there on the ground we may find some dry twigs, lying half-buried in the soil. Once they too were branches of the vine; now they lie there withered and dead, a perfect image of barrenness: `apart from Me, you can do nothing'" ([St] J. Escriva, "Friends of God", 254).
The life of union with Christ is necessarily something which goes far beyond one's private life: it has to be focused on the good of others; and if this happens, a fruitful apostolate is the result, for "apostolate, of whatever kind it be, must be an overflow of the interior life" ([St] J. Escriva, "Friends of God", 239). The Second Vatican Council, quoting this page from St. John, teaches what a Christian apostolate should be: "Christ, sent by the Father, is the source of the Church's whole apostolate. Clearly then, the fruitfulness of the apostolate of lay people depends on their living union with Christ; as the Lord Himself said: `He who abides in Me, and I in him, he it is that bears much fruit, for apart from Me you can do nothing'. This life of intimate union with Christ in the Church is maintained by the spiritual helps common to all the faithful, chiefly by the active participation in the Liturgy. Laymen should make such a use of these helps that, while meeting their human obligations in the ordinary conditions of life, they do not separate their union with Christ from their ordinary life; but through the very performance of their tasks, which are God's will for them, actually promote the growth of their union with Him" ("Apostolicam Actuositatem", 4).
6. If a person is not united to Christ by means of grace he will ultimately meet the same fate as the dead branches--fire. There is a clear parallelism with other images our Lord uses--the parables of the sound tree and the bad tree (Matthew 7:15-20), the dragnet (Matthew 13:49-50), and the invitation to the wedding (Matthew 22:11-14), etc. Here is how St. Augustine comments on this passage: "The wood of the vine is the more contemptible if it does not abide in the vine, and the more glorious if it does abide....For, being cut off it is profitable neither for the vinedresser nor for the carpenter. For one of these only is it useful--the vine or the fire. If it is not in the vine, it goes to the fire; to avoid going to the fire it must be joined to the vine" ("In Ioann. Evang.", 81, 3).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.