The Word, Church, and Sacraments In Protestantism and Catholicism is a gem of a little book by Fr. Louis Bouyer, C.O. I’d highly recommend it for anyone who has a deep love for the spiritual riches present in both Protestant and Catholic traditions. Interestingly, he wrote this book in 1960–four years before the Church’s Decree on Ecumenism (Unitatis Redintegratio) was promulgated.
At the very conclusion, he speaks on what Christian unity means:
“For this unity to be fully realized, we do not ask our separated brethren to forego any part of what is positive and authentic in their great religious insights. On the contrary, we ask them to draw from these fearlessly all their logical conclusions. We ask them to realize that the Church does not oppose them in order to deny or to minimize what they rightly hold to be essential, but rather to safeguard the full reality, in a completeness that no truth of Christianity can possess except in the one, whole Body of Christ.
Nonetheless, to have the right to ask them this effort, we Catholics have to make one of our own…We must, in the first place, understand them and, before hastening to say ‘no’ to what is erroneous, however extensive, be prompt to say ‘yes’, frankly and unreservedly, to all truths, even and especially if they are ones to which we habitually pay little attention. Afterward, no doubt, but only afterward, comes the corresponding duty to help our brethren to sort out for themselves the great truths they have rediscovered from the errors involved with them. This second task, certainly, is no less essential to a real ‘ecumenism’ than the first; but to enter on it without regard for the first, without working at the first, would be to toil in vain.
This being so, it is equally essential for us to give a clear, positive witness to the truth that we chance, or rather have the undeserved grace, to possess. But this witness must be given to the whole truth and not merely to certain aspects of it to which we habitually restrict ourselves out of habit, facility, or mere indolence. Since there is but one Christian truth, Catholic truth in the real sense of the word [universal], that is, a truth complete and whole, it is by making this effort of total fidelity to our own patrimony, and making it fully, that we shall be best prepared to make the required effort of opening our minds to the truths rightly cherished by our separated brethren.
But we must be fully aware that all that has been said will be of no effect unless accompanied by an effort, constantly renewed, to bring our own practice, our daily life, into harmony with the doctrine we profess.” (p. 89-91)
This is how the New Evangelization can further Jesus’ call “that all may be one” (John 17:21).