Question Third. On Sins Against Faith

25. What sins there are against faith. One can sin against faith by omission, namely by omitting: a. due diligence in learning the truths of faith necessary to be known; b. the prescribed act of faith; c. the profession of faith prescribed either by God or by the Church; and by commission, by placing an action prohibited by the precept of faith. A sin of commission can be committed by excess and by defect. By excess one sins through credulity, by which, without sufficient reason, something is rashly believed as revealed by God which has not been revealed, as if someone lightly believes private revelations; yet this sin does not exceed venial sin. By defect one sins by external or internal denial of faith, which happens by infidelity, apostasy, heresy, doubt concerning faith, and communication in sacred things with unbelievers and heretics.