Perhaps you are referring to St. Peter Faber, was one of St. Ignatius of Loyola’s companions and helped found the Jesuit order. He said:
“[It] is essential that whoever desires to be useful to heretics in our day should both nourish in himself a great affection for them and show it in action, removing from his own mind those unfavourable imaginations which make us think less well of them.” (Instructions How to Deal with Heretics, as it appears in The life of Blessed Peter Favre by Giuseppe Boero, Chapter 13)
“The next thing is, to win their goodwill and inclinations to such an extent that they may reciprocate our kind feelings and think well of us. This may easily be done by speaking to them affectionately, and dwelling in familiar conversations on those points only on which they agree with us, avoiding everything like a dispute, in which one side always assumes an air of superiority, and shows contempt of the other. Those subjects should be first chosen in which there is a sympathy and union of wills, rather than those which tend to disunite them by opposition of opinion.” (ibid.)
Is that the quotation you are referring to, or is there another? If there is, I would love to find it.