John 15:5

"He that abideth in Me, and I in him." Seest thou that the Son contributeth not less than the Father towards the care of the disciples? The Father purgeth, but He keepeth them in Himself. The abiding in the root is that which maketh the branches to be fruit-bearing. For that which is not purged, if it remain on the root, bears fruit, though perhaps not so much as it ought; but that which remains not, bears none at all. But still the "purging" also hath been shown to belong to the Son, and the "abiding in the root," to the Father, who also begat the Root. Seest thou how all is common, both the "purging," and the enjoying the virtue which is from the root?

(Tract. lxxxi. 3) But lest any should suppose that a branch could bring forth a little fruit of itself, He adds, For without Me ye can do nothing. He does not say, ye can do little. Unless the branch abides in the vine, and lives from the root, it can bear no fruit whatever. Christ, though He would not be the vine, except He were man, yet could not give this grace to the branches, except He were God.

Ponder again and again what the Truth has still further to say: "I am the vine," He adds, "ye are the branches: he that abideth in me, and I in him, the same bringeth forth much fruit; for without me ye can do nothing." For just to keep any from supposing that the branch can bear at least some little fruit of itself, after saying, "the same bringeth forth much fruit," His next words are not, Without me ye can do but little, but "ye can do nothing." Whether then it be little or much, without Him it is impracticable; for without Him nothing can be done. For although, when the branch beareth little fruit, the husbandman purgeth it that it may bring forth more; yet if it abide not in the vine, and draw its life from the root, it can bear no fruit whatever of itself. And although Christ would not have been the vine had He not been man, yet He could not have supplied such grace to the branches had He not also been God. And just because such grace is so essential to life, that even death itself ceases to be at the disposal of free-will, He adds, "If any one abide not in me, he shall be cast forth as a branch, and wither; and they shall gather him, and cast him into the fire, and he is burned." The wood of the vine, therefore, is in the same proportion the more contemptible if it abide not in the vine, as it is glorious while so abiding; in fine, as the Lord likewise says of them in the prophet Ezekiel, when cut off, they are of no use for any purpose of the husbandman, and can be applied to no labor of the mechanic. The branch is suitable only for one of two things, either the vine or the fire: if it is not in the vine, its place will be in the fire; and that it may escape the latter, may it have its place in the vine.

All the fruit of good works proceeds from this root. He who hath delivered us by His grace, also carries us onward by his help, so that we bring forth more fruit. Wherefore He repeats, and explains what He has said: I am the vine, ye are the branches. He that abideth in Me, by believing, obeying, persevering, and I in Him, by enlightening, assisting, giving perseverance, the same, and none other, bringeth forth much fruit.