The most recent Papal documents dealing with the problem of Christian unity are very careful to remind us of what the Catholic doctrine on the subject of the Church and its unity is. The fear is expressed in them, in several places, lest an anxiety to promote Christian unity, commendable doubtless but not sufficiently clear-sighted, should lead in the mind of some to a certain playing down of the essential teaching of the Church in this matter. we have in mind some expressions now current that these documents quite evidently avoid for fear that their use might lead to a dangerous confusion in the minds of their reader. It will be useful, therefore, with a view to securing complete fidelity to the full meaning of these documents, to examine here one of these expressions, order to settle accurately, with exact theological terminology, in what sense and context it may be used, and when, on the contrary, it should not.