By St. Cyril of Alexandria.
Paul bears witness to the fact that we achieve bodily union with Christ to the extent that we partake of his holy flesh. About this great mystery he says This that has now been revealed through the Spirit to his holy apostles and prophets was unknown to any men in past generations: it means that pagans now share the same inheritance, that they are parts of the same body, and that the same promise has been made to them, in Jesus Christ.
If we are all the same body with one another in Christ – not just with one another, but with him who, through communion with his flesh, is actually within us – are we not then all of us clearly one with one another and one with Christ? For Christ is the bond that unites us, being at once God and Man.
Following the same line of thought, we can say this about spiritual unity: we all receive one and the same Spirit, the one Holy Spirit, I mean the Holy Spirit. So in a way we are blended together with one another and with God. Even though we are many individuals and Christ, the Spirit of the Father and his own Spirit, dwells in each one of us individually, still the Spirit is really one and indivisible. And so that one Spirit binds together the separated spirits of each one of us so that we are seen to be one, together in Christ.
Just as the power of Christ’s holy flesh makes into one body everyone in whom it exists, in the same way the Spirit of God, being indivisible, ties together the spirits in which it dwells.
Again, Paul emphasized this point: Bear with one another charitably, in complete selflessness, gentleness and patience. Do all you can to preserve the unity of the Spirit by the peace that binds you together. There is one Body, one Spirit, just as you were all called into one and the same hope when you were called. There is one Lord, one faith, one baptism, and one God who is Father of all, over all, through all and within all. As the one Spirit abides in us, the one God and Father will be with us through the Son, leading those who share the Spirit into unity with each other and with himself.
There is another way to show that we are united through sharing in the Holy Spirit. If we abandon living as mere animals and surrender ourselves wholly to the laws of the Spirit, it is surely beyond question that by effectively denying our own life and taking upon ourselves the transcendent likeness of the Holy Spirit who is joined unto us, we are practically transformed into another nature. We are no longer mere men, but sons of God and citizens of Heaven, through becoming partakers of the divine nature.
We are all, therefore, one in the Father, and the Son, and the Holy Spirit; one because we have the same relationship, one because we live the same life of righteousness, and one in receiving the holy flesh of Christ and in sharing the one Holy Spirit.
From: Commentary on the Gospel of St John