15. Great is the error of those who approach the sacrament of the Eucharist relying on this, that they have confessed, that they are not conscious of any mortal sin, that they have sent their prayers on ahead and made preparations; all these eat and drink judgment to themselves. But if they believe and trust that they will attain grace, then this faith alone makes them pure and worthy.
The "this" here is contrition (from #14). If one did not have true contrition for their sins when they confessed them, then the confession would be invalid. If the confession is invalid, then they would still have that/those sin/s upon their souls. If one approaches the Eucharist with unforgiven sins upon them, then they do so in the unworthy manner St. Paul refers to in 1 Cor. 11:27. To approach the Eucharist in such a manner - even if you "believe" in this "faith alone" does the exact opposite of making one "pure and worthy," for it brings upon them "the judgment."