The following is taken from the Catechetical Instructions of St. Thomas Aquinas.
THE NINTH ARTICLE: "I Believe in the Holy Catholic Church."
We see that in a man there are one soul and one body; and of his body there are many members. So also the Catholic Church is one body and has different members. The soul which animates this body is the Holy Spirit.1 Hence, after confessing our faith in the Holy Ghost, we are bid to believe in the Holy Catholic Church. Thus, in the Symbol it is said, "the Holy Catholic Church."
It must be known that "church" is the same as assembly.2 So, the Holy Church is the same as the assembly of the faithful, and every Christian is a member of this Church, of which it is written: "Draw near to Me, ye unlearned; and gather yourselves together into the house of discipline."3
The Church has four essential conditions, in that she is one, holy, catholic, and strong and firm.4
THE UNITY OF THE CHURCH
Of the first, it must be known that the Church is one. Although various heretics have founded various sects, they do not belong to the Church, since they are but so many divisions. Of her it is said: "One is My dove; My perfect one is but one."5 The unity of the Church arises from three sources:
(1) the unity of faith. All Christians who are of the body of the Church believe the same doctrine. "I beseech you . . . that you all speak the same thing and that there be no schisms among you."6 And: "One Lord, one faith, one baptism;"7
(2) the unity of hope. All are strengthened in one hope of arriving at eternal life. Hence, the Apostle says: "One body and one Spirit, as you are called in one hope of your calling;"8
(3) the unity of charity. All are joined together in the love of God, and to each other in mutual love: "And the glory which Thou hast given Me, I have given them; that they may be one, as We also are one."9 It is clear that this is a true love when the members are solicitous for one another and sympathetic towards each other: "We may in all things grow up in Him who is the head, Christ. From whom the whole body, being compacted, and fitly joined together, by what every joint supplieth, according to the operation in the measure of every part, maketh increase of the body unto the edifying of itself in charity."10 This is because each one ought to make use of the grace God grants him, and be of service to his neighbor. No one ought to be indifferent to the Church, or allow himself to be cut off and expelled from it; for there is but one Church in which men are saved, just as outside of the ark of Noah no one could be saved.
THE HOLINESS OF THE CHURCH
Concerning the second mark, holiness, it must be known that there is indeed another assembly, but it consists of the wicked: "I hate the assembly of the malignant."11 But such a one is evil; the Church of Christ, however, is holy: "For the temple of God is holy, which you are."12 Hence, it is said: "the Holy Church."
The faithful of this Church are made holy because of four things: (1) Just as a church is cleansed materially when it is consecrated, so also the faithful are washed in the blood of Christ: "Jesus Christ . . . who hath loved us and washed us from our sins in His own blood."13 And: "That He might sanctify the people by his blood, suffered without the gate."14 (2)
Just as there is the anointing of the church, so also the faithful are anointed with a spiritual unction in order to be sanctified. Otherwise they would not be Christians, for Christ is the same as Anointed. This anointing is the grace of the Holy Spirit: "He that confirmeth us with you in Christ and that hath anointed us, is God."15 And: "You are sanctified . . . in the name of our Lord Jesus Christ."16 (3) The faithful are made holy because of the Trinity who dwells in the Church; for wheresoever God dwells, that place is holy. "The place whereon thou standest is holy."17 And: "Holiness becometh Thy house, O Lord."18 (4) Lastly, the faithful are sanctified because God is invoked in the Church: "But Thou, O Lord, art among us, and Thy name is called upon by us; forsake us not."19 Let us, therefore, beware, seeing that we are thus sanctified, lest by sin we defile our soul which is the temple of God: "Know you not that you are the temple of God and that the Spirit of God dwelleth in you? But if any man violate the temple of God, him shall God destroy."20
THE CATHOLICITY OR UNIVERSALITY OF THE CHURCH
The Church is Catholic, that is, universal. Firstly, it is universal in place, because it is worldwide. This is contrary to the error of the Donatists.21 For the Church is a congregation of the faithful; and since the faithful are in every part of the world, so also is the Church: "Your faith is spoken of in the whole world."22 And also: "Go ye into the whole world and preach the gospel to every creature.23 Long ago, indeed, God was known only in Judea; now, however, He is known throughout the entire world. The Church has three parts: one is on earth, one is in heaven, and one is in purgatory. Secondly, the Church is universal in regard to all the conditions of mankind; for no exceptions are made, neither master nor servant, neither man nor woman: "Neither bond nor free; there is neither male nor female."24. Thirdly, it is universal in time. Some have said that the Church will exist only up to a certain time. But this is false, for the Church began to exist in the time of Abel and will endure up to the end of the world: "Behold, I am with you all days, even to the consummation of the world."25 Nay more, even after the end of the world, it will continue to exist in heaven.
THE APOSTOLICITY OF THE CHURCH
The Church is firm. A house is said to be firm if it has a solid foundation. The principal foundation of the Church is Christ: "For other foundation no men can lay but that which is laid, which is Christ Jesus."26 The secondary foundation, however, is the Apostles and their teaching. Therefore, the Church is firm. It is said in the Apocalypse that the city has "twelve foundations," and therein were "written the names of the twelve Apostles."27 From this the Church is called Apostolic. Likewise, to indicate this firmness of the Church St. Peter is called the crowning head.28
The firmness of a house is evident if, when it is violently struck, it does not fall. The Church similarly can never be destroyed, neither by persecution nor by error. Indeed, the Church grew during the persecutions, and both those who persecuted her and those against whom she threatened29 completely failed: "And whosoever shall fall upon this stone, shall be broken; but on whomsoever it shall fall, it shall grind him to powder."30 As regards errors, indeed, the more errors arise, the more surely truth is made to appear: "Men corrupt in mind, reprobate in faith; but they shall proceed no further."31
Nor shall the Church be destroyed by the temptations of the demons. For she is like a tower towards which all flee who war against the devil: "The name of the Lord is a strong tower."32 The devil, therefore, is chiefly intent on destroying the Church, but he will not succeed, for the Lord has said: “The gates of hell shall not prevail against it."33
This is as if He said: "They shall make war against thee, but they shall not overcome thee." And thus it is that only the Church of Peter (to whom it was given to evangelize Italy when the disciples were sent to preach) was always firm in faith. On the contrary, in other parts of the world there is either no faith at all or faith mixed with many errors. The Church of Peter flourishes in faith and is free from error. This, however, is not to be wondered at, for the Lord has said to Peter: "But I have prayed for thee, that thy faith fail not; and thou, being once converted, confirm thy brethren."34
1. "For as the body is one and hath many members; and all the members of the body, whereas they are many, yet are one body, so also is Christ. For in one Spirit were we all baptized into one body. . . . For the body also is not one member, but many" (I Cor., xii. 12-14). For St. Paul's admirable description of the Church, Christ's mystical body, see all of this chapter.
2. "The word "ecclesia" (church) which is borrowed by the Latins from the Greek has been applied since the preaching of the Gospel to sacred things. The word "ecclesia" (church) means a calling forth, but writers afterwards used it to mean a council or assembly. . . . However, in the ordinary sense used in the Scriptures, the word was afterwards used to designate the Christian society only, and the assemblies of the faithful: that is, of those who were called by faith to the light of truth, and the knowledge of God" ("Roman Catechism," Ninth Article, 2).
3. Ecclus., li. 31.
4. "The distinctive marks of the Church are also to be made known to the faithful that they thus may be able to appreciate the extent of the blessing conferred by God on those who have the happiness to be born and educated in her fold" ("Roman Catechism," "loc. cit.," 2).
5. Cant., vi. 8.
6. I Cor., i. 10.
7. Eph., iv. 5.
8. "Ibid." 4.
9. John, xvii. 22.
10. Eph., iv. 15-16.
11. Ps. xxv. 5.
12. I Cor., iii. 17.
13. Apoc., i. 5.
14. Heb., xiii. 12.
15. II Cor., i. 2
16. I Cor., vi. 11.
17. Josue, v. 16; cfr. also Gen., xxviii. 16.
18. Ps. xcii, 5.
19. Jerem., xiv. 9.
20. I Cor., iii. 16-17. "It should not be considered surprising that the Church, although among her children are many sinners, is called holy. For as those who profess any art, even though they may violate its rules, are still artists, so the faithful, although offending in many things and violating the promises which they have made, are still called holy, because they are made the people of God, and are consecrated to Christ by baptism and faith" ("Roman Catechism," "loc. cit.," 15).
21. A sect which existed chiefly in Africa for about a century (311-411).
22. Rom., i. 8.
23. Mark. xvi. 15.
24. Gal., iii. 28.
25. Matt., xxviii. 20.
26. I Cor., iii. 11.
27. Apoc., xxi. 14.
28. As it is spoken of by Our Lord: "And I say to thee that thou art Peter; and upon this rock I will build My Church, and the gates of hell shall not prevail against it" (Matt., xvi. 18).
29. That is, enemies of the Church who in one or other ways resisted the authority or teachings of the Church.
30. Matt., xxi. 44.
31. Tim., iii. 8.
32. Prov., xviii. 10.
33. Matt., xvi. 18.
34. Luke, xxii. 32.