"it is a lack of faith not to have faith in the faithful."

Pseudo Jacopino di Francesco

"Just as the wickedness of heretics should be crushed by the zeal of true faith, so the integrity of genuine confession [(verae confessionis . . . integritas)] should be embraced.  For if trust in one who confesses faithfully [(fideliter confitenti)] is despised, the faith of all men is made doubtful, and deadly sins arise from inconsiderate strictness. . . .  Let us consider this, dearest brother, with great care, and not allow anyone who truly professes [(veraciter profitentem)] the Catholic faith to be afflicted under the pretext of heresy, in case (Heaven forbid!) we allow heresy to grow stronger under a pretext of correcting it."

     Gregory the Great, Letter 6.15 to John, Bishop of Constantinople, September 595, trans. John R. C. Martyn (The letters of Gregory the Great, vol. 2, books 5-9, Medieval sources in translation 40 (Toronto:  Pontifical Institute of Medieval Studies, 2004), 413.

"it is a lack of faith not to have faith in the faithful [(Nam nullus ambigit infidelitatem esse fidem fidelibus non habere)]. . . .  For not trusting someone who confesses truthfully [(veraciter profitenti)] is not purging heresy, but creating it [(non est heresem purgare, sed facere)]."

      Gregory the Great, Letter 6.16 to Maurice, Augustus, September 595, trans. John R. C. Martyn (The letters of Gregory the Great, vol. 2, books 5-9, Medieval sources in translation 40 (Toronto:  Pontifical Institute of Medieval Studies, 2004), 415.
     Latin from MGH (not dMGH) here.

     I was put onto these passages by Blaise Pascal, Provincial letters no. 17, trans. M’Crie, who quotes them in defense of the Jansenists somewhat differently:

If . . . we refuse to believe a confession made in conformity to the sentiments of the Church, we cast a doubt over the faith of all Catholics whatsoever. . . .  your object is to make these persons heretics in spite of themselves; because to refuse to credit those who testify by their confession that they are in the true faith, is not to purge heresy, but to create it—hoc non est haeresim purgare, sed facere.