St Photios (later known as Photios the Great - Patriarch of Constantinople) was born around 820 AD to holy parents, who were confessors of the Faith. His parents were persecuted for defending icons against the iconoclasts and were exiled from Constantinople. His greatness was not only due to his defence of Orthodoxy against heretical papal practices, but also connected to his love and meekness. He vigorously opposed the addition of the filioque clause to the Nicene - Constantinopolitan Creed, and wrote On the Mystagogy of the Holy Spirit to preserve "the purity of our religion" and to hinder "those who chose to promote any other definition of dogma than the unanimous and common faith of the pious". This treatise became the pattern for all subsequent Byzantine anti-Latin polemics.
The filioque doctrine, espoused by Western Christians, has its source from Augustine of Hippo (359 - 432 AD). Augustine had a fertile imagination, who could not shake off the Platonic influence of his youth. The doctrine of a 'double procession of the Holy Spirit' was first adopted in the West at the Synod of Toledo (447 AD), which appears to have followed Augustine's teachings. This addition was forbidden by the Fourth Ecumenical Council (451 AD). Here is the origin of the problem that was to agitate the Church for a thousand years. Contentions that the filioque has Biblical foundations have yet to be demonstrated.
St. Photios was forced to become Patriarch of Constantinople, however he took his calling seriously and at once set to work as a man of God. One of his activities was to correct the error of pope Nicholas of Rome who enslaved the people of the West with threats of condemnation to hell for disobedience to the pope. Holy Photios wrote to Nicholas "Nothing is dearer that the Truth". In the same letter he noted "It is truly necessary that we observe all things, but above all, that which pertains to matters of Faith, in which but a small deviation represents a deadly sin".
As a Father of the Church, St. Photios was also known for his brilliance and for his missionary zeal. He blessed St Cyril in his work of developing an alphabet for the Slavonic people, and for the later work of St Cyril and his brother St Methodios as missionaries to the Slavonic people.
That, even as the Son is proclaimed by the Sacred Oracles to be begotten of the Father alone, so also is the Holy Spirit proclaimed by theology to proceed from this same and only cause. He is, however, said to be of the Son, since he is of one essence with Him and is sent through Him.
St. Photios reposed in the Lord in 891 AD, and his feast day is celebrated on February 6.
The life of St Photios is available in the French language here.
More information available here.