But this philosophical demonstration, however cogent in itself, puts into play most abstract and difficult notions. There is in man, I think, another certitude of his own immortality, a certitude born of the instinctive, non-conceptual functioning of intelligence in the obscure experience each man has of his own Self intent on existence and struggling with Time, which must be, in some unknown way, superior to the stream of Time and of phenomena. The funeral rites of primitive man witness to this natural, instinctive, pre-philosophical knowledge of immortality, which took in him forms adapted to the state of a thought dominated by mythical imagination. (But, as Lecomte du Nouÿ observed, the more a belief is ancient in mankind, the more we should pay attention to it, assuming that, queer and coarse as its various expressions might have been, human nature is somewhat a warrant of its value.)
And this natural, instinctive, pre-philosophical knowledge of immortality exists, I think, in each one of us in an unconscious or preconscious state, inexpressible in terms of conceptual reason, but rooted in vital experience. It exists in those whose conceptual reason denies immortality as in those whose conceptual reason affirms it. And without such instinctive certitude we could not understand the manner in which men both cherish their life and expose it to all kinds of dangers. Yet when belief in immortality is conscious and reasoned out, it makes man more ready, as far as his intellect and reflection are concerned, to give his own life for the sake of superior causes, of freedom, of truth. thus Gandhi stated that it was necessary for his disciples "to believe that death does not mean cessation of the struggle but a culmination", because they knew that the soul survives the body.
Finally the immortality of the human soul is a tenet of religious faith, particularly of Christian faith, which insists on the supernatural destiny of the soul, called to see God face to face. As a matter of fact, religious faith, here as with some other great philosophical problems, has strengthened human reason in the very grasping of truths which human reason is capable of attaining of itself, but at the risk of error.
As concerns your more particular questions, I do not see any basic contradiction between the Greek and Socratic belief that there is an immortal mind in a mortal body, and the emphasis on unity of life which we find in ancient Hebrews, - on the condition that the immortal mind in question not be regarded (in the Platonic or Hindu manner) as a spirit prisoner in an alien place or a bird in a cage, but as a spirit substantially one with the body it animates (according to the Aristotelian notions of "form" or "entelechy", which Thomas Aquinas has made classical in Catholic theology). Man is both in nature and transcending nature, that's the mystery which struck Pascal so deeply in the human being.
Resurrection is no more irrational than any datum received by faith from divine revelation. Reason sees that nothing is more impossible to nature. But reason knows that nothing is impossible to God, except absurdity; and there is no absurdity in the fact of a separate soul being united to matter anew.
You ask me whether my attitude on immortality changed as I grew older? In growing older one feels more inclined to deny the reality of old age, except as regards the body, and more convinced that human life is at the same time so serious a thing and so foolishly used that nothing on earth would make sense were not an incomprehensible mystery of mercy involved in life beyond the grave.