Rorate Caeli

Lacordaire: : A Protestant is his own unity

You will perhaps ask why God has left so many questions open to discussion. You might as well ask why God has not revealed everything. 
Now God has revealed the principles in order to serve as foundations, He has not done exactly the same with the consequences, in order to give our liberty play, like a mother who holds her child up by leading strings, but is delighted to see him try and walk like a man. You must bear in mind too, that this infallibility may, at any moment, transfer ideas from the realm of opinion to that of dogma, and consequently from the free to the necessary order. A simple decision of the Church works this change, and she never withholds that decision from the human race in case of need.

Seated in the midst of minds, unchangeable like God, whose Spirit she has, the Church diffuses in a wonderful manner light and heat, drawing to herself every soul of good will, judging human ideas by the standard of divine ones, and welding together in admirable peace the very differences she allows to exist among her childrenTheir liberty gives her no uneasiness, for she knows on the one hand, the point at which she will check them, and on the other she is certain they will stop at her bidding. It is much the same kind of feeling as that of God about the ocean.

On the contrary, Protestant liberty recognises no bounds, and is destructive of all unity. ... He [the Protestant] is his own unity: in other words, his unity is something essentially variable, a cloud, a dew drop. His individuality itself does not constitute unity: he is alone without the possibility of being one; God is one without being able to be alone, and His Church in like manner.

Henri-Dominique Lacordaire
Letter to a young man 
Solesmes, June 24, 1838

[Trans. Fr. James Trenor]