Second Council of Lyon
– The Procession of the Holy Spirit –
We confess faithfully and devoutly that the Holy Spirit proceeds eternally from the Father and Son, not as from two principles, but from one, not by two spirations, but by one only. This the holy Roman Church, the mother and teacher of all the faithful, has so far professed, preached, and taught; this she continues to hold, to preach, to profess, and to teach. This is the unchangeable and true doctrine of the orthodox Fathers and Doctors, both Latin and Greek. However, some have fallen into various errors out of ignorance of the above indisputable truth.
Therefore, in order to forestall such errors, with the approval of the holy council, We condemn and disapprove those who presume to deny that the Holy Spirit proceeds eternally from Father and Son or who rashly dare to assert that the Holy Spirit proceeds from Father and Son as from two principles, not from one.
The Council of Florence
– Decree for the Greeks –
[The procession of the Holy Spirit.] Therefore, in the name of the Holy Trinity, Father, Son, and Holy Spirit, with the approval of this sacred universal Council of Florence, we define that this truth of faith must be believed and received by all Christians, and so all must profess that the Holy Spirit is eternally from the Father and the Son, and he has his essence and his subsistent being at once from the Father and the Son, and he proceeds eternally from both as from one principle and one spiration [cf. Council of Lyon II: *850].
We declare that when the holy Doctor and Fathers say that the Holy Spirit proceeds from the Father through the Son, this tends toward that understanding which signifies that the Son, like the Father, is also what the Greeks call “cause” and the Latins “principle” of the subsistence of the Holy Spirit.
And since the Father himself has given to his only begotten Son, in generating him, all that the Father has except being the Father, the Son himself eternally has from the Father, from whom he is eternally generated, precisely this: that the Holy Spirit proceeds from the Son.
Moreover, we define that the explanatory words “Filioque” have been added in the Creed legitimately and with good reason for the sake of clarifying the truth and under the impact of a real need at the time.
*Taken from Denzinger’s Compendium of Creeds, Definitions, and Declarations on Matters of Faith and Morals
– Lucas G. Westman