Truly, I believe a strong sense of awareness and not 'belonging' in a mortal body happened often among humans over millennia. I suspect Justin Martyr sensed the same and was seeking understanding which he found in Christ and, as you say, saw Christ also fulfilling classical philosophy.
Indeed. One might almost say that this sense of "not belonging in a mortal body" is probably more the rule than the exception in human historical experience. It is, however, just not the common sense nowadays; i.e., in "modernity," or the "post-modern world."
The first documentary evidence on this question of which I am aware dates back to ~2,000 B.C. Egypt, to an anonymous text known as "Dispute of a Man, Who Contemplates Suicide, with His Soul." In this "dialogue," the Man is disgusted, and in despair, because he finds himself living in a society so corrupt, so disordered, so oppressive to his own moral and cosmological understandings, that he can't bear to live in it anymore. Hence the contemplation of suicide — a solution that he thinks will spare him from the general corruption of society, and of the prospect of his further contributing to it. He sees his existential position in terms of a stark choice: either of "going along to get along" with the evil of his age, or self-annihilation. The Man thinks the latter may be the better choice.
But his Soul counsels him: You must not take your own life; for it is a precious gift of the gods.
In his brilliant essay "Immortality: Experience and Symbol," Eric Voegelin delineates the problem:
The Man is driven to despair by the troubles of a disordered age and wants to cast off a life that has become senseless; the Soul is introduced as the speaker who militates against the decision.... [T]he struggle between Man and his Soul is concerned with the idea of life as a gift of the gods. Since life is not a man's property to be thrown away when it becomes burdensome but an endowment to be treated as a trust under all conditions, the Soul can point to the command of the gods and the wisdom of the sages which both prohibit the shortening of the alloted span. But Man knows how to plead: the disintegration of order, both personal and public, in the surrounding society deprives life of any conceivable meaning, so that exceptional circumstances will justify a violation of the rule before the gods....Notwithstanding the Man's arguments — who is "pitted against the disorder of society" — his Soul counsels that "he can emerge as victor from the struggle because he carries in himself the full reality of order...."
There is much more to the "Dispute" than this; but I'll leave it there for now. I just cite it here to show that problems of body and soul and of human existence and immortality have been in the forefront of the most consuming, vital questions that human beings have universally asked from as far back as the historical records go.
Obviously, the "Dispute" is pre-Christian by date. But that is not to say that it undermines the greatest truths about Man as part and participant in the Great Hierarchy of Being as revealed — and fulfilled — in Judeo-Christian theology.
I so agree with you: "...Justin Martyr sensed the same and was seeking understanding which he found in Christ and, as you say, saw Christ also fulfilling classical philosophy." To which I would add: And also Christ Who fulfills the main insights of an anonymous work dating from the Egypt of the First Intermediate Period.
The Logos — God's Word of Creation — has been "in the world" ( so to speak) from the very Beginning....
Just some thoughts FWTW. Thank you ever so much for writing, dearest sister in Christ!