Are separated Christians or non-believers–or anyone, really–just making an error? Or is there some inherent blindness to the truth? I think one might be hard-pressed to suggest every errant person is always blameless or always responsible. In any event, it does the believer no harm to work with that person at all. In fact is may well be an opportunity for evangelization. This is aligned with the inherent optimism of the conciliar outlook: good things will happen because God gives the opportunities.
158. It is always perfectly justifiable to distinguish between error as such and the person who falls into error—even in the case of (those) who err regarding the truth or are led astray as a result of their inadequate knowledge, in matters either of religion or of the highest ethical standards. (Someone) who has fallen into error does not cease to be a (person). (They) never forfeit … personal dignity; and that is something that must always be taken into account. Besides, there exists in (a person’s) very nature an undying capacity to break through the barriers of error and seek the road to truth. God, in His great providence, is ever present with His aid. Today, maybe, a (person) lacks faith and turns aside into error; tomorrow, perhaps, illumined by God’s light, (they) may indeed embrace the truth.
Catholics who, in order to achieve some external good, collaborate with unbelievers or with those who through error lack the fullness of faith in Christ, may possibly provide the occasion or even the incentive for their conversion to the truth.