Libertas §20

²⁰ But, assuredly, of all the duties which man has to fulfill, that, without doubt, is the chiefest and holiest which commands him to worship God with devotion and piety. This follows of necessity from the truth that we are ever in the power of God, are ever guided by His will and providence, and, having come forth from Him, must return to Him. Add to which, no true virtue can exist without religion, for moral virtue is concerned with those things which lead to God as man's supreme and ultimate good; and therefore religion, which (as St. Thomas says) "performs those actions which are directly and immediately ordained for the divine honor",(7) rules and tempers all virtues. And if it be asked which of the many conflicting religions it is necessary to adopt, reason and the natural law unhesitatingly tell us to practice that one which God enjoins, and which men can easily recognize by certain exterior notes, whereby Divine Providence has willed that it should be distinguished, because, in a matter of such moment, the most terrible loss would be the consequence of error. Wherefore, when a liberty such as We have described is offered to man, the power is given him to pervert or abandon with impunity the most sacred of duties, and to exchange the unchangeable good for evil; which, as We have said, is no liberty, but its degradation, and the abject submission of the soul to sin.