The atoning sacrifice of Christ is the foundation of the Church. If that is so than the Church is comprised of those that have received through faith in Christ the Saving Grace that he has offered. That faith, therefore, is what the Church is based on - the foundation of the Church. I know you would like to reference the RC Church as that foundation.
You say "the atoning sacrifice of Christ is the foundation of the Church." Then you say "If so..." This proves it is but your own surmise. I'm not buying it. Please *prove* - from Scripture alone, as that is doubtless your worldview - your contention in your first sentence, something hard to do when you yourself use the qualifier "if" in the very next sentence.