That is a great question! I imagine one could argue both sides. To argue that Plato's Phaedo is a rationalist text and not a religious one, I would point to these ideas:
- There is no component of faith in the argument that Socrates advances; he doesn't ask his followers to believe in the existence of the soul without proof. He attempts to provide proof by relying on (among other things) the concept of Forms. One might argue that his Forms, as objects that are intangible and unrecognizable to the senses, are something he asks listeners to take on faith. He argues, however, that Forms are distinguishable by a rational intellectual process. However, in true rationalist spirit, he doesn't believe that their existence can be recognized with the senses; only the mind can perceive the forms, and only through the exercise of reason (reason trumps experience as evidence):
“He attains to the purest knowledge of them who goes to each with the mind alone, not introducing or intruding in the act of thought, sight or any other sense together with reason, but with the very light of the mind in her own clearness searches into the very truth of each.”
- Whether you find the above very plausible or not, Socrates does at least attempt to justify his beliefs with evidence and a logical process, using phrases like “sufficient proof” and “logical necessity.” So he's not looking for faith.
- Plato's Socrates also doesn't ask his listeners to worship anything in particular. (In fact many believe that was the reason for his execution; he refused to worship the Greek gods.) There are no rituals associated with the theory he puts forward; one does not need to do anything to be "saved" or live on. The soul is immortal in all people. That said, Socrates does say that the souls of those who are wicked or tied too closely to their physical body may find themselves too heavy; dragged back down to earth, they will be reincarnated as animals. Only those who have committed to the way of philosophy, understanding, and rationality may go on to a blissful afterlife and escape reincarnation. (Sounds pretty religion-y, no?)
In short, it looks like there's a compelling argument for either side. Perhaps most tend to view Plato's Phaedo as a rationalist text at least in part because that was how Plato himself identified it: as a philosophical, not a religious work. Whether his methods and logic hold up to modern standards or not, he meant philosophy to be an escape from deception and a way to "teach the soul to rely on its own intellectual resources," placing reason over faith and emotion (eNotes). Plato/Socrates says of philosophy,
“she will calm passion, and follow reason, and dwell in the contemplation of her, beholding the true and divine.”