[W]e must say this, that if we take “letter” in the sense of the literal formulation of a commandment and “spirit” in the sense of the intention and meaning of the commandment, it is never possible to fulfill the spirit without fulfilling the letter when moral commandments including an absolute veto are in question. For example, an absolute veto is found in the moral commandments: “Thou shalt not commit adultery,” “Thou shalt not curse,” or “Thou shalt not sacrifice to idols or deny God.” Here it is absolutely impossible to claim that someone could ever depart from the letter without violating the spirit, that is, without sinning.
It makes no sense to say that although someone committed adultery in the literal sense of the word he remains true to the spirit of the commandment—if we take “spirit” in the sense of the meaning and intention of the commandment. The formulation here is such that “spirit” necessarily includes the letter, so that the possibility of any departure from the letter without violating the spirit is excluded regardless of the circumstances.
Dietrich von Hildebrand, True Morality and Its Counterfeits, New York:David McKay Company (1955).