All who receive the sacred flesh of Christ are united with him as members of his body. This is the teaching of Saint Paul when he speaks of the mystery of our religion that was hidden from former generations, but has now been revealed to the holy apostles and prophets by the Spirit; namely, that the Gentiles are joint-heirs with the Jews, that they are members of the same body, and that they have a share in the promise made by God in Christ Jesus.
If, in Christ, all of us, both ourselves and he who is within us by his own flesh, are members of the same body, is it not clear that we are one, both with one another and with Christ? He is the bond that unites us, because he is at once both God and man.
With regard to our unity in the Spirit, we may say, following the same line of thought, that all of us who have received one and the same Spirit, the Holy Spirit, are united intimately, both with one another and with God. Taken separately, we are many, and Christ sends the Spirit, who is both the Father’s Spirit and his own, to dwell in each of us. Yet that Spirit, being one and indivisible, gathers together those who are distinct from each other as individuals, and causes them all to be seen as a unity in himself. Just as Christ’s sacred flesh has power to make those in whom it is present into one body, so the one, indivisible Spirit of God, dwelling in all, causes all to become one in spirit.
Therefore, Saint Paul appeals to us to bear with one another charitably, and to spare no effort in securing, by the bonds of peace, the unity that comes from the Spirit. There is but one body and one Spirit, just as there is but one hope held out to us by God’s call. There is one Lord, one faith, one baptism, one God and Father of all, who is above all, and works through all, and is in all. If the one Spirit dwells in us, the one God and Father of all will be in us, and he, through his Son, will gather together into unity with one another and with himself all who share in the Spirit.
There is also another way of showing that we are made one by sharing in the Holy Spirit. If we have given up our worldly way of life and submitted once for all to the laws of the Spirit, it must surely be obvious to everyone that by repudiating, in a sense, our own life, and taking on the supernatural likeness of the Holy Spirit, who is united to us, our nature is transformed so that we are no longer merely men, but also sons of God, spiritual men, by reason of the share we have received in the divine nature. We are all one, therefore, in the Father and the Son and the Holy Spirit. We are one in mind and holiness, we are one through our communion in the sacred flesh of Christ, and through our sharing in the one Holy Spirit.
Saint Cyril of Alexandria, bishop