Today is the Feast of St Photios of Constantinople (+ ca. 893). See OrthodoxWiki for more.
Photios is best known for his rejection of the novel 9th century doctrine that the Spirit of God proceeds “from the Father and the Son” (qui ex Patre Filioque procedit).
The Nicene Creed sung in the Divine Liturgy confesses only that the Spirit proceeds from the Father (τὸ ἐκ τοῦ Πατρὸς ἐκπορευόμενον).
PRECISELY because the Spirit does not proceed “also from the Son”, nor beyond the Son, but remains as the bond between Father and Son, he gathers us together in the Son whose face is turned always toward the Father.
The communion with us and among us which he effects is not something apart from the Son and the Trinity.
Through the Spirit we who have been united in the Son have a filial relation to the Father, not in exactly the same manner, obviously, as natural sons begotten by the Father, but in the manner of sons adopted through the Spirit, which is to say, we have been gathered into one under the same overshadowing presence of the Spirit in whom is also the Son and through whom the Son is bound to the Father.
We are related among ourselves as brothers, and Jesus Christ is Brother to us all in our midst.
Fr Dumitru Staniloae, “Theology And The Church” (SVS Press 1980), Chapter 2, p. 63.
SEE now! what is so good, or what so pleasant, as for brethren to dwell together? It is as ointment on the head, that ran down to the beard, even the beard of Aaron; that ran down to the fringe of his clothing. As the dew of Hermon, that comes down on the mountains of Sion: for there, the Lord commanded the blessing, even life for ever.