In mine (Library of Christian Classics, edited by John T. McNeil), it is in Book 3, chp 3, #23, last two sentences.
"For when he (Paul) says that they who willing, sin after having received knowledge of the truth have no sacrifice left for them (Heb 10:26), he does not deny that Christ is a continual sacrifice to atone for the iniquities of the saints. Almost the whole letter eloquently proclaims this, in explaining Christ's priesthood. But he says that no other sacrifice remains when his has been rejected. Moreover, it is rejected when the trurth of the gospel is expressly denied."
Unless I misunderstand him, which would not be surprising. . .and he means continual atonement.
Continuing sacrifice (in the Mass) is Catholic doctrine.