Today’s Catechism sections describe the personal and community aspects of the faith, including the unity of the faith.

Article 2

WE BELIEVE

166 Faith is a personal act - the free response of the human person to the initiative of God who reveals himself. But faith is not an isolated act. No one can believe alone, just as no one can live alone.

You have not given yourself faith as you have not given yourself life. the believer has received faith from others and should hand it on to others. Our love for Jesus and for our neighbor impels us to speak to others about our faith. Each believer is thus a link in the great chain of believers. I cannot believe without being carried by the faith of others, and by my faith I help support others in the faith.

167 "I believe" (Apostles' Creed) is the faith of the Church professed personally by each believer, principally during Baptism. "We believe" (Niceno-Constantinopolitan Creed) is the faith of the Church confessed by the bishops assembled in council or more generally by the liturgical assembly of believers. "I believe" is also the Church, our mother, responding to God by faith as she teaches us to say both "I believe" and "We believe".

I. "Lord, Look Upon the Faith of Your Church"

168 It is the Church that believes first, and so bears, nourishes and sustains my faith. Everywhere, it is the Church that first confesses the Lord: "Throughout the world the holy Church acclaims you", as we sing in the hymn Te Deum; with her and in her, we are won over and brought to confess: "I believe", "We believe". It is through the Church that we receive faith and new life in Christ by Baptism. In the Rituale Romanum, the minister of Baptism asks the catechumen: "What do you ask of God's Church?" and the answer is: "Faith." "What does faith offer you?" "Eternal life."54

169 Salvation comes from God alone; but because we receive the life of faith through the Church, she is our mother: "We believe the Church as the mother of our new birth, and not in the Church as if she were the author of our salvation."55 Because she is our mother, she is also our teacher in the faith.

II. The Language of Faith

170 We do not believe in formulae, but in those realities they express, which faith allows us to touch. "The believer's act [of faith] does not terminate in the propositions, but in the realities [which they express]."56 All the same, we do approach these realities with the help of formulations of the faith which permit us to express the faith and to hand it on, to celebrate it in community, to assimilate and live on it more and more.

171 The Church, "the pillar and bulwark of the truth", faithfully guards "the faith which was once for all delivered to the saints". She guards the memory of Christ's words; it is she who from generation to generation hands on the apostles' confession of faith.57 As a mother who teaches her children to speak and so to understand and communicate, the Church our Mother teaches us the language of faith in order to introduce us to the understanding and the life of faith.

III. Only One Faith

172 Through the centuries, in so many languages, cultures, peoples and nations, the Church has constantly confessed this one faith, received from the one Lord, transmitted by one Baptism, and grounded in the conviction that all people have only one God and Father.58 St. Irenaeus of Lyons, a witness of this faith, declared:

173 "Indeed, the Church, though scattered throughout the whole world, even to the ends of the earth, having received the faith from the apostles and their disciples. . . guards [this preaching and faith] with care, as dwelling in but a single house, and similarly believes as if having but one soul and a single heart, and preaches, teaches and hands on this faith with a unanimous voice, as if possessing only one mouth."59

174 "For though languages differ throughout the world, the content of the Tradition is one and the same. the Churches established in Germany have no other faith or Tradition, nor do those of the Iberians, nor those of the Celts, nor those of the East, of Egypt, of Libya, nor those established at the centre of the world. . ."60 The Church's message "is true and solid, in which one and the same way of salvation appears throughout the whole world."61

175 "We guard with care the faith that we have received from the Church, for without ceasing, under the action of God's Spirit, this deposit of great price, as if in an excellent vessel, is constantly being renewed and causes the very vessel that contains it to be renewed."62

St. Irenaeus describes the Unity of the Catholic Faith in his great work, “Against Heresies” (Book 1, Ch. 10):

Unity of the faith of the Church throughout the whole world.

1. The Church, though dispersed through our the whole world, even to the ends of the earth, has received from the apostles and their disciples this faith: [She believes] in one God, the Father Almighty, Maker of heaven, and earth, and the sea, and all things that are in them; and in one Christ Jesus, the Son of God, who became incarnate for our salvation; and in the Holy Spirit, who proclaimed through the prophets the dispensations of God, and the advents, and the birth from a virgin, and the passion, and the resurrection from the dead, and the ascension into heaven in the flesh of the beloved Christ Jesus, our Lord, and His [future] manifestation from heaven in the glory of the Father to gather all things in one, Ephesians 1:10 and to raise up anew all flesh of the whole human race, in order that to Christ Jesus, our Lord, and God, and Savior, and King, according to the will of the invisible Father, every knee should bow, of things in heaven, and things in earth, and things under the earth, and that every tongue should confess Philippians 2:10-11 to Him, and that He should execute just judgment towards all; that He may send spiritual wickednesses, Ephesians 6:12 and the angels who transgressed and became apostates, together with the ungodly, and unrighteous, and wicked, and profane among men, into everlasting fire; but may, in the exercise of His grace, confer immortality on the righteous, and holy, and those who have kept His commandments, and have persevered in His love, some from the beginning [of their Christian course], and others from [the date of] their repentance, and may surround them with everlasting glory.

2. As I have already observed, the Church, having received this preaching and this faith, although scattered throughout the whole world, yet, as if occupying but one house, carefully preserves it. She also believes these points [of doctrine] just as if she had but one soul, and one and the same heart, and she proclaims them, and teaches them, and hands them down, with perfect harmony, as if she possessed only one mouth. For, although the languages of the world are dissimilar, yet the import of the tradition is one and the same. For the Churches which have been planted in Germany do not believe or hand down anything different, nor do those in Spain, nor those in Gaul, nor those in the East, nor those in Egypt, nor those in Libya, nor those which have been established in the central regions of the world. But as the sun, that creature of God, is one and the same throughout the whole world, so also the preaching of the truth shines everywhere, and enlightens all men that are willing to come to a knowledge of the truth. Nor will any one of the rulers in the Churches, however highly gifted he may be in point of eloquence, teach doctrines different from these (for no one is greater than the Master); nor, on the other hand, will he who is deficient in power of expression inflict injury on the tradition. For the faith being ever one and the same, neither does one who is able at great length to discourse regarding it, make any addition to it, nor does one, who can say but little diminish it.

3. It does not follow because men are endowed with greater and less degrees of intelligence, that they should therefore change the subject-matter [of the faith] itself, and should conceive of some other God besides Him who is the Framer, Maker, and Preserver of this universe, (as if He were not sufficient for them), or of another Christ, or another Only-begotten. But the fact referred to simply implies this, that one may [more accurately than another] bring out the meaning of those things which have been spoken in parables, and accommodate them to the general scheme of the faith; and explain [with special clearness] the operation and dispensation of God connected with human salvation; and show that God manifested longsuffering in regard to the apostasy of the angels who transgressed, as also with respect to the disobedience of men; and set forth why it is that one and the same God has made some things temporal and some eternal, some heavenly and others earthly; and understand for what reason God, though invisible, manifested Himself to the prophets not under one form, but differently to different individuals; and show why it was that more covenants than one were given to mankind; and teach what was the special character of each of these covenants; and search out for what reason God Romans 11:32 has concluded every man in unbelief, that He may have mercy upon all; and gratefully describe on what account the Word of God became flesh and suffered; and relate why the advent of the Son of God took place in these last times, that is, in the end, rather than in the beginning [of the world]; and unfold what is contained in the Scriptures concerning the end [itself], and things to come; and not be silent as to how it is that God has made the Gentiles, whose salvation was despaired of, fellow-heirs, and of the same body, and partakers with the saints; and discourse how it is that this mortal body shall put on immortality, and this corruptible shall put on incorruption; 1 Corinthians 15:54 and proclaim in what sense [God] says, That is a people who was not a people; and she is beloved who was not beloved; Hosea 2:23; Romans 9:25 and in what sense He says that more are the children of her that was desolate, than of her who possessed a husband. Isaiah 54:1; Galatians 4:27 For in reference to these points, and others of a like nature, the apostle exclaims: Oh! The depth of the riches both of the wisdom and knowledge of God; how unsearchable are His judgments, and His ways past finding out! Romans 11:33 But [the superior skill spoken of] is not found in this, that any one should, beyond the Creator and Framer [of the world], conceive of the Enthymesis of an erring Æon, their mother and his, and should thus proceed to such a pitch of blasphemy; nor does it consist in this, that he should again falsely imagine, as being above this [fancied being], a Pleroma at one time supposed to contain thirty, and at another time an innumerable tribe of Æons, as these teachers who are destitute of truly divine wisdom maintain; while the Catholic Church possesses one and the same faith throughout the whole world, as we have already said.

Footnotes

54 Roman Ritual, Rite of Baptism of Adults.
55 Faustus of Riez, De Spiritu Sancto 1, 2: PL 62, II.
56 St. Thomas Aquinas, STh II-II, 1,2, ad 2.
57 ⇒ I Tim 3:15; Jude 3.
58 Cf. ⇒ Eph 4:4-6
59 St. Irenaeus, Adv. haeres. I, 10, 1-2: PG 7/1, 549-552.
60 St. Irenaeus, Adv. haeres. I, 10, 1-2: PG 7/1, 552-553.
61 St. Irenaeus, Adv. haeres. 5, 20, I: PG 7/2, 1177.
62 St. Irenaeus, Adv. haeres. 3, 24, I: PG 7/1, 966.