"in such a way that each one of the faithful can be certain"

"the Deputation de fide is not of the mind that this word should be understood in a juridical sense (Lat. in sensu forensi) so that it only signifies putting an end to controversy which has arisen in respect to heresy and doctrine which is properly speaking de fide.  Rather, the word 'defines' signifies that the Pope directly and conclusively pronounces his sentence about a doctrine which concerns matters of faith or morals and does so in a way that each one of the faithful can be certain of the mind of the Apostolic See, of the mind of the Roman Pontiff; in such a way, indeed, that he or she knows for certain that such and such a doctrine is held to be heretical, proximate to heresy, certain or erroneous, etc., by the Roman Pontiff.  Such, therefore, is the meaning of the word 'defines.'"

Bishop Vincent Ferrer Gasser, relatio of  16 July 1870; The gift of infallibility:  the official relatio on infallibility of Bishop Vincent Gasser at Vatican Council I (11 July 1870), trans. Rev. James T. O'Connor (Boston, MA:  St. Paul Editions, 1986), 74.