The Catholic Church is the most unique institution that has ever existed. Only she claims to be the authentic Bride of Christ. Through her, we live and have life in sanctifying grace, for it is to her that Our Lord left the means of salvation, which are found through the Sacraments.
“It is so evident from the clear and frequent testimonies of Holy Writ that the true Church of Jesus Christ is one, that no Christian can dare to deny it. But in judging and determining the nature of this unity many have erred in various ways. Not the foundation of the Church alone, but its whole constitution, belongs to the class of things effected by Christ’s free choice. For this reason the entire case must be judged by what was actually done. We must consequently investigate not how the Church may possibly be one, but how He, who founded it, willed that it should be one. But when we consider what was actually done we find that Jesus Christ did not, in point of fact, institute a Church to embrace several communities similar in nature, but in themselves distinct, and lacking those bonds which render the Church unique and indivisible after that manner in which in the symbol of our faith we profess: ‘I believe in one Church.’ ‘The Church in respect of its unity belongs to the category of things indivisible by nature, though heretics try to divide it into many parts… We say, therefore, that the Catholic Church is unique in its essence, in its doctrine, in its origin, and in its excellence… Furthermore, the eminence of the Church arises from its unity, as the principle of its constitution – a unity surpassing all else, and having nothing like unto it or equal to it’ (S. Clemens Alexandrinus, Stronmatum lib. viii., c. 17). For this reason Christ, speaking of the mystical edifice, mentions only one Church, which he calls His own – ‘I will build my church;’ any other Church except this one, since it has not been founded by Christ, cannot be the true Church.” ~Pope Leo XIII, “Satis Cognitum”, 1896 A.D.
The third mark of the Church is that she is Catholic; that is, universal. And justly is she called Catholic, because, as St. Augustine says, she is diffused by the splendor of one faith from the rising to the setting sun. Unlike states of human institutions of the sects of heretics, she is not confined to any one country or class of men, but embraces within the amplitude of her love all mankind, whether barbarians or Scythians, slaves or freemen, male or female… Moreover to this Church, built on the foundation of the apostles and prophets belong all the faithful who have existed from Adam to the present day, or who shall exist, in the profession of the true faith, to the end of time… She is also called universal, because all who desire eternal salvation must cling to and embrace her, like those who entered the ark to escape perishing in the flood. This (note of catholicity), therefore, is to be taught as a most reliable criterion, by which to distinguish the true from a false Church.” ~Catechism of the Council of Trent