Vatican Council II 

Constitution on the Church in the modern world « Gaudium et spes », § 92 (©Libreria Vaticana editrice)

"Whoever is not against you is for you"

By virtue of her mission to shed on the whole world the radiance of the Gospel message, and to unify under one Spirit all men of whatever nation, race or culture, the Church stands forth as a sign of that brotherhood which allows honest dialogue and gives it vigor.

Such a mission requires in the first place that we foster within the Church herself mutual esteem, reverence and harmony, through the full recognition of lawful diversity. Thus all those who compose the one People of God, both pastors and the general faithful, can engage in dialogue with ever abounding fruitfulness. For the bonds which unite the faithful are mightier than anything dividing them. Hence, let there be unity in what is necessary; freedom in what is unsettled, and charity in any case. Our hearts embrace also those brothers and communities not yet living with us in full communion; to them we are linked nonetheless by our profession of the Father and the Son and the Holy Spirit, and by the bond of charity... We think cordially too of all who acknowledge God, and who preserve in their traditions precious elements of religion and humanity. We want frank conversation to compel us all to receive the impulses of the Spirit faithfully and to act on them energetically.

For our part, the desire for such dialogue, which can lead to truth through love alone, excludes no one, though an appropriate measure of prudence must undoubtedly be exercised. We include those who cultivate outstanding qualities of the human spirit, but do not yet acknowledge the Source of these qualities. We include those who oppress the Church and harass her in manifold ways. Since God the Father is the origin and purpose of all men, we are all called to be brothers. Therefore, if we have been summoned to the same destiny, human and divine, we can and we should work together without violence and deceit in order to build up the world in genuine peace.