The Orthodox seem to err in a) attaching undue importance to that particular line of the Credo being a direct quote from the Bible, at the expense of fully explaining the matter, and b) jumping through hoops to develop a theology that explains away Gal 4:6, Rom 8:9, Phil 1:19, and Christ's sending of the Holy Ghost in Luke 24:49; John 15:26; 16:7; 20:22; Acts 2:33; Titus 3:6. Just because John 15:26 states the Holy Ghost proceeds from the Father, it doesn't by extension require that the Holy Ghost not also proceed from the Son. Scripture refers to the Holy Ghost as both the Spirit of the Father AND the Spirit of the Son, and Christ's own words in John 16:15, in my opinion, settle the matter. ALL that is the Father's is the Son's, in a way we are not capable of understanding.
I would like to find that paper "Spirit of God, Spirit of Christ," but I would point out that the Catholic Church is not a member of the WCC (although certain heretical sects claiming to the Catholic Church are members), and hence, I regard the paper with skepticism until I see which, if any, Catholic theologians contributed.