Then you must not have come across his argument that souls can "endure as long as God wills them to endure" (perseverant autem quoadusque eas Deus et esse, et perseverare voluerit).
In other words, his argument is that the soul is not itself a life: it must be given life.And that which is given is outside of the self, its nature. Only God is Life itself (Adversus haereses II, 34).
Like I said, you need to get your facts straight, HD.
Clement of Alexandria, who was a Platonist, admits that the soul was not immortal "by nature" (hinc apparet quoniam non est naturaliter anima incorruptibilis, Adumbrationes, I Petri 1:9) .
+Jerome states "I do not say, indeed, that all souls die."
If Man from the beginning had chosen things immortal, in obedience to God's commandments, he would have been rewarded with immortality and have become God, "an adoptive God," deus assumptus, Theos anadihthis (+Theophilus of Antioch, Ad Autolycum II, 24 and 27).
"The soul is not in itself immortal, O Greeks, but mortal. Yet it is possible for it not to die" (Tatian, Oratio ad Graecos, 13).
Blessed Augustine qualifies the immortality of the soul to be given by God: Anima hominis immortalis est secundum quendam modum suum; non enim omni modo sicut Deus (Epist. VFF, ad Hieronymum (and is therefore not by its nature itself life), and he also states in two instances (Jo., tr. 23, 9; cf. De Trinitate, 19.15, and De Civ. Dei, 19.3) that the soul is mortal according to the mutability of this life (mortalis in quantum mutabilis).
+John of Damascus states that even Angels are not immortal by nature, but only by grace (Exact Exposition of the Orthodox Faith II, 3)
[T]hat "intellectual beings are not immortal by nature" [but only by the grace of God] (+Sophronius, the Patriarch of Jerusalem, VI Ecumenical Council, 681 AD)
I would encourage folks to go and read, not only the scriptures, but the church fathers. They are rather clear and to suggest they did not believe in the immortality of the soul is just plain wrong.
I will add I am a little confused in the Orthodox position. On the one hand you're saying that the soul is mortal and then on the other hand you're saying all souls will be before God living in some sort of separated thing. The Orthodox doctrinal position on this matter is inconsistent, is not in compliance with scripture, goes against the majority of the church fathers, and are against the teachings of the Church prior to the Orthodox schism of the 1000+ century. I would say that it isn't the Protestants who are holding the heretical view this time.