cyril_alexandriaI am the true Vine, and My Father is the Husbandman (John 15:1).

He wishes to show us that it behoves us to love, to hold fast to our love towards Him, and how great a gain we shall have from our union with Him, when He says that He is the Vine, by way of illustration.

Those who are united and fixed and rooted in a manner in Him, and who are already partakers in His nature through their participation in the Holy Spirit are branches.

For it is His Holy Spirit Which has united us with the Saviour Christ, since connexion with the Vine produces a choice of those things which belong to It, and our connexion with It holds us fast.

From a firm resolve in goodness we proceed onward by faith, and we become His people, obtaining from Him the dignity of Sonship. For according to the holy Paul, He that is joined unto the Lord is one Spirit. 

[…] For we are begotten of Him and in Him in the Spirit, to produce the fruits of life; not the old life of former days, but that which consists in newness of faith and love towards Him.

And we are preserved in our hold on this life by clinging as it were to Him, and holding fast to the holy commandment given to us, and by making haste to preserve the blessing of our high birth.

We do this by refusing to grieve in any way whatever the Holy Spirit That has taken up His abode in us, by Whom God is conceived to dwell in us.

For in what manner we are in Christ and He in us the wise John will show us when he says: Hereby we know that we are in Him and He in us, by the Spirit Which He gave us.

And again John says, Hereby know we that we are in Him; he that saith he abideth in Him ought himself also to walk even as He walked. 

And he makes this even clearer to his hearers by the words, He that keepeth His commandments abideth in Him, and He in him. 

[…] The root of the vine ministers and distributes to the branches the enjoyment of its own natural and inherent qualities.

So also the Only-begotten Word of God imparts to the Saints as it were an affinity to His own nature and the nature of God the Father, by giving them the Spirit, insomuch as they have been united with Him through faith and perfect holiness.

And He nourishes them in piety, and works in them the knowledge of all virtue and good works.

Cyril of Alexandria (c. 376-444): Commentary on John, Book 10 (on John 15:1ff).