Taken from Medulla S. Thomae Aquinatis which arranges various short texts from St. Thomas Aquinas' corpus based on the liturgical year. I will be posting the day's meditation each day and will be bringing it into print once I'm through it.
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On Dwelling in Christ
As the branch cannot bear fruit of itself, unless it abide in the vine, so neither can you, unless you abide in me. (John 15:4)
I. Here, he shows that being united to Christ is necessary in order to bear fruit. It is as saying, I say that you should abide in me so that you can bear fruit, because just as the branch literally, a material branch, cannot bear fruit of itself unless it abide in the vine, from whose roots sap ascends to give life to the branches, so neither can you bear fruit unless you abide in me. Thus, being united to Christ is the reason why someone bears fruit. And so of those who are not united to Christ we read: what return did you get from the things of which you are now ashamed? (Rom 6:21); the company of the godless is barren (Job 15:34).
His example is apt because I am the vine, you the branches. It is like saying: the relation between you and me is like that of branches to the vine. We read of these branches: it sent out its branches to the sea (Ps 80:11).
II. Here he shows that being united to Christ is efficacious. It is not only necessary for a person to abide in me in order to bear fruit, it is also efficacious, because he who abides in me, by believing, obeying and persevering, and I in him, by giving enlightenment, help, and perseverance, he it is and not another, bears much fruit. Such persons bear a threefold fruit in this life. The first is that they avoid sin. Second, they are eager to accomplish works of holiness: the return you get is sanctification (Rom 6:22). Third, they are eager for the progress of others: the earth will be filled with the fruit of your works (Ps 104:13). They also produce a fourth fruit, but in eternal life. Eternal life is the last and perfect fruit of our labors
The reason for this efficacy is because without me you can do nothing. Our Lord says that without him we cannot do anything great, nor anything small, indeed, we cannot do anything at all. This is not surprising because neither does God do anything without him: without him was made nothing that was made (John 1:3). For our works are either from the power of nature or from divine grace. If they are from the power of nature, then, since every action of nature is from the Word of God, no nature can act to do anything without him. If our works are from the power of grace, then, since he is the author of grace, because grace and truth came by Jesus Christ (John 1:17), then it is obvious that no meritorious work can be done without him: not that we are capable of thinking anything of ourselves as originating from ourselves; our capability is from God (2 Cor 3:5). Therefore, if we cannot even think without it coming from God, much less can we do anything else.
(Selections from Ioan.C15.L1)