From what has been said concerning the Primacy of the Roman Pontiff, it is clear that that Church alone which acknowledges this Primacy is the true Church of Christ. Nevertheless, as Catholics in their discussions with Protestants are accustomed to waive this proof, and to appeal to such marks of the true Church as are admitted by both sides, we shall now proceed to speak of these.

It should be noted that, though the Church possesses many properties, not all of these are marks, in the technical sense of the word. Marks are those properties or signs by which she may be distinguished from other bodies. She alone possesses the marks; other bodies may possess certain of her properties. Thus, visibility is one of her properties; yet this may belong to heretical sects. Unity, Holiness, Catholicity, and Apostolicity are marks, and are found in her alone. On the subjects dealt with in this chapter, see Franzelin, De Ecclesia, sect. iv.; Palmieri, De Ecclesia, p. 27 sqq.; De Rom. Pont., Append., p. 677 sqq.; Murray, De Ecclesia, cap. iv. sqq.; Newman, Angl. Diff., I. p. 229 sqq.; Billot, De Eccl., p. 128 sqq.; Turmel, op. cit., p. 117 sqq.; Atzberger, op. cit., sect. 331 sqq.