Philo as Originator of Immortal Soul

Phaedrus, 250c:

That was the ultimate vision, and we saw it in pure light because we were pure ourselves, not buried in the thing we are carrying around now, which we call a body, locked in it like an oyster in its shell.

Cratylus, 400c:

Thus some people say that the body (sōma) is the tomb (sēma) of the soul, on the grounds that it is entombed in its present life, while others say that it is correctly called ‘a sign’ (sēma) because the soul signifies whatever it wants to signify by means of the body. I think it is most likely the followers of Orpheus who gave the body its name, with the idea that the soul is being punished for something, and that the body is an enclosure or prison in which the soul is securely kept (sōzetai) –as the name ‘sōma’ itself suggests–until the penalty is paid; for, on this view, not even a single letter or the word needs to be changed.

 Gorgias, 293a:

and we really, it may be, are dead; in fact I once heard sages say that we are now dead, and the body is our tomb, and the part of the soul in which we have desires is liable to be over-persuaded and to vacillate to and fro, and so some smart fellow, a Sicilian, I daresay, or Italian, made a fable in which—by a play of words—he named this part, as being so impressionable and persuadable, a jar, and the thoughtless he called uninitiate:

Note also the definition Plato gives to death, which many Christian groups nowadays have adopted:

Phaedo, 64c:

Let us then,” said he, “speak with one another, paying no further attention to them. Do we think there is such a thing as death?” “Certainly,” replied Simmias. “We believe, do we not, that death is the separation of the soul from the body, and that the state of being dead is the state in which the body is separated from the soul and exists alone by itself and the soul is separated from the body and exists alone by itself? Is death anything other than this?” “No, it is this,” said he. “Now, my friend, see if you agree with me; 

[NOTE: Title may be somewhat misleading given that other philosophers like Pindar and Pythagoras had already  alluded to an immortal soul. But it was Philo who popularized and who brought it into its classic expression that we know now.]