As discussed previously, starting in about 879, Saint Methodius was accused of having a heterodox teaching concerning the Holy Spirit. Pope John VIII implicitly denied the Filioque when defending Methodius, claiming:

Accordingly, we questioned this Methodius, your venerable archbishop, in the presence of our brother bishops, whether he adheres to the creed of faith [fidei symbolum] in the orthodox way and during the sacred liturgical rites sings as is held by the Holy Roman Church [i.e. without the Filioque] and as was announced and established by holy six universal councils of holy fathers according to the evangelical authority of our Lord Christ. He thus declared that he believes and sings [the Psalms] according to the evangelical and apostolic teaching, as the Holy Roman Church teaches and as was established by the fathers. (Letter 11/Letter 293)

And so, John VIII affirms that Methodius had the same Creed as Rome and the Ecumenical Councils (which lacked the Filioque) and that his “teaching” is the same as “the Holy Roman Church teaches.” This means, his exegesis of the Creed, without the Filioque, matched whatever Rome’s was–which we know from Anastasius the Librarian commenting shortly before on this question was not the Double Procession doctrine dogmatized later by Lyons and Florence.

Here, I will provide some quotations contextualizing the above.

In Letter 238, the King of Moravia alleged “we have heard that Methodius, your archbishop, ordained by our predecessor, that is, Hadrian, the pope, and directed to you, teaches otherwise than what he has professed to believe in words and letters before the apostolic see.”

Letter 239 is from Pope John VIII to Methodius, where he summons him to Rome to give an account “whether you hold and preach as you have promised to believe in the words and letters of the holy Roman Church, or not, so that we may truly know your teaching.”

Letter 339 repeats threateningly, “we reminded [monuimus] you to follow the doctrine of the holy Roman Church, according to the probable tradition of the holy Fathers, and both the symbol and the right faith that we submitted to you to teach and preach.”

It is unclear, being that without a doubt Rome had a Creed without the Filioque, what precisely motivates Rome’s exacting stance towards Methodius. It is possible that Rome was trying to draw a dichotomy between the Pneumatological teachings of Photius (which are more minimalistic than the Damscene or Augustine) and themselves, perhaps as a way of laying the groundwork of accusing Photius of a Pneumatologically reductionist heresy. That’s my speculation and I’ll leave it there.