2.13 How can I be sure that the Church is telling the truth?
Jesus himself instituted the Church, with Peter and the other Apostles and their successors as her leaders. He called the Church the “bulwark of the truth” (I Tim. 3:15) I Tim. 3:15: If I am delayed, you may know how one ought to behave in the household of God, which is the church of the living God, the pillar and bulwark of the truth.. To ensure that the Apostles could pass on the truth of the faith without error, Jesus gave them the Holy Spirit.
Also today the Holy Spirit helps the Church to understand the faith better. Therefore, and because it is not just a human institution, you can trust that the Church always tells the truth about God.
Why is the Church apostolic?
The Church is apostolic in her origin because she has been built on “the foundation of the Apostles” (Ephesians 2:20). She is apostolic in her teaching which is the same as that of the Apostles. She is apostolic by reason of her structure insofar as she is taught, sanctified, and guided until Christ returns by the Apostles through their successors who are the bishops in communion with the successor of Peter. [CCCC 174]
In what way is the Church holy?
The Church is holy insofar as the Most Holy God is her author. Christ has given himself for her to sanctify her and make her a source of sanctification. The Holy Spirit gives her life with charity. In the Church one finds the fullness of the means of salvation. Holiness is the vocation of each of her members and the purpose of all her activities. The Church counts among her members the Virgin Mary and numerous Saints who are her models and intercessors. The holiness of the Church is the fountain of sanctification for her children who here on earth recognize themselves as sinners ever in need of conversion and purification. [CCCC 165]
The preaching of the Church is everywhere consistent, and continues in an even course, and receives testimony from the prophets, the Apostles, and all the disciples... and that well-grounded system which tends to man's salvation, namely, our faith. [St. Irenaeus, Against heresies, Bk 3, Chap. 24 (MG 7, 966)]