Adam and Eve in the state of original innocence possessed preternatural gifts: "bodily immortality, infused knowledge, and immunity from concupiscence."¹
St. Thomas Aquinas addresses the question "Whether the first man knew all things?,"² saying:
…the first man was established by God in such a manner as to have knowledge of all those things for which man has a natural aptitude.
The first man had knowledge of all things by divinely infused species.
Thus, Adam knew his soul was immortal.
Also, Pope Leo X (1513-1521) wrote in his bull Regiminis
, Session VIII, Dec. 19, 1513, of the 5th Lateran Council that:
…we condemn and reject all who assert that the intellectual soul is mortal…
I understand Flimzy's objection in the comments below that "'all those things for which man has a natural aptitude' is a far cry from 'everything naturally knowable'" to mean that although Adam may have been potentially able to know all things naturally knowable, whether he actually did is unknown. Regarding this, St. Thomas, before the passage I quoted above, wrote:²
In the natural order, perfection comes before imperfection, as actuality precedes potentiality; for whatever is in potentiality is made actual only by something actual. And since God created things not only for their own existence, but also that they might be the principles of other things; so creatures were produced in their perfect state to be the principles as regards others. Now man can be the principle of another man, not only by generation of the body, but also by instruction and government. Hence, as the first man was produced in his perfect state, as regards his body, for the work of generation, so also was his soul established in a perfect state to instruct and govern others.Thus, Adam had actual knowledge of "all those things for which man has a natural aptitude."
- Parente, Pietro, Antonio Piolanti, and Salvatore Garofalo. Dictionary of Dogmatic Theology. Milwaukee: Bruce, 1951. p. 228.
- Summa Theologica I q. 94 a. 3 c. & ad 1