In Aquinas' Expositio in Symbolum Apostolorum (Explanation of the Apostles' Creed), his explanation in Article 9 of the Church being "catholic" includes this statement:
Nam aliqui dixerunt, quod Ecclesia debet durare usque ad certum tempus. Sed hoc est falsum: quia haec Ecclesia incepit a tempore Abel, et durabit usque ad finem saeculi…
Some have said that the Church will exist only up to a certain time. But this is false, for the Church began to exist in the time of Abel and will endure up to the end of the world…
I presume that the Abel he is referring to is the one of Cain and Abel in Genesis. Unfortunately, Aquinas doesn't explain in this text his reasoning for why the Church began to exist at this time, as opposed to with Adam and Eve, with any of the Jewish heroes of faith, or with Jesus.
On what basis does Thomas Aquinas say that the Church existed since the time of Abel?