A friend of mine who is pursuing a masters in theology recently commented that because human souls are eternal, it means we have always existed somewhere, even before our physical creation. Is that accurate, though? I know our souls are immortal once created and will therefore continue to exist eternally, but I had never heard it said that we had always existed.
The idea of the preexistence of human souls is found in the writings of Plato. We also find it in the writings of the Hellenistic Jewish philosopher, Philo of Alexandria. The idea was also embraced by the 3rd century Christian writer, Origen, though this was not believed in general by the Christian Church.
St. Thomas Aquinas offers four arguments against the pre-existence of the soul in question 3, article 10, of his QUAESTIONES DISPUTATAE DE POTENTIA DEI. You can read the arguments at the following link: dhspriory.org/thomas/QDdePotentia3.htm#3:10
The 2nd Council of Constantinople, in its anathemas against Origen, anathematized anyone that believes in the pre-existence of the human soul.
Catechism of the Catholic Church, in paragraphs 362-368, adopts the understanding of the soul as the “form” of the body, which implies the soul comes into existence at the time a human being (body and soul) comes into existence.