John 15:4
“As the branch cannot bear fruit of itself except it abide in the vine, so neither can he who abides not in Me.”
For that they might not be separated from Him by timidity, He fastens and glues to Himself their souls slackened through fear, and holds out to them good hopes for the future. For the root remains, but to be taken away, or to be left, belongs to the branches. Then having urged them on in both ways, by things pleasant and things painful, He requires first what is to be done on our side.
Source: Homilies on the Gospel of John (New Advent)