Today in Pacem in Terris, we read we can cooperate fruitfully with separated Christians or even non-believers:
157. The principles We have set out in this document take their rise from the very nature of things. They derive, for the most part, from the consideration of natural (human) rights. Thus the putting of these principles into effect frequently involves extensive co-operation between Catholics and those Christians who are separated from this Apostolic See. It even involves the cooperation of Catholics with (those) who may not be Christians but who nevertheless are reasonable (people), and (individuals) of natural moral integrity. “In such circumstances they must, of course, bear themselves as Catholics, and do nothing to compromise religion and morality. Yet at the same time they should show themselves animated by a spirit of understanding and unselfishness, ready to co-operate loyally in achieving objects which are good in themselves, or conducive to good.”(Mater et Magistra)
Catholic Sensibility 