The Church is the Pillar and Foundation of the Truth
". . . but if I am delayed, I write so that you may know how you ought to conduct yourself in the house of God, which is the church of the living God, the pillar and ground of the truth." - St. Paul to young pastor Timothy (1 Timothy 3:15)
It's interesting that when you ask most Protestant Christians this question: "What is 'the pillar and foundation of the truth,'" they will almost always say, "The Bible!" or "God's Word!" St. Paul's on the other hand says that the church that is the pillar and foundation of the truth. While the Bible is certainly Divinely inspired, inerrant truth from the pens of human authors, it is also the product and possession of the church, thus, what St. Paul is teaching here to young Timothy is that God speaks His Divinely inspired truth in and through His church and dispenses His grace in and through His church.
God gives gifts to the church, one of which are teachers, who are called and blessed the church with the wisdom and discernment to rightly handle the word of truth. Some of these men were doctors of the church (e.g.,John Chrysostom, Basil the Great, Gregory of Nazianzus, Athanasius of Alexandria, Ambrose, Jerome, and Bede the Venerable, etc.). These great teachers, like their teachers, did not dive into their own heads and rely on their fallen intellects to inform them about theological matters; instead, they were like wise builders building a house dug deep into rock, where the flood waters of heresy and schism beat vehemently and could not shake it, for it was founded on the rock. This Rock is Christ, His gospel, and the apostolic doctrines enshrined in the teachings of the Scriptures as understood and taught by the one, holy, catholic, and apostolic church.
The Church is mystically the Body of Christ
I now rejoice in my sufferings for you, and fill up in my flesh what is lacking in the afflictions of Christ, for the sake of His body, which is the church, (Colossians 1:24) - St. Paul to The Church in Colossae.
God's people are called the body of Christ, with Christ being the head (Colossians 1:18a). As the head of the church, Jesus is our High Priest, our King, and our Savior. As High Priest, He intercedes for us in matters of salvation. As our King, the Lord Jesus Christ rules over us; subjects in His holy kingdom--the church. As Savior, Jesus is the spotless Lamb of God who takes away the sins of the world.
The Church is the Israel of God
And as many as walk according to this rule, peace and mercy be upon them, and upon the IsraelofGod. (Galatians 6:16) - St. Paul to The Church in Galatia.
God interacted with His people (Israel) in the Old Testament age through covenants. A covenant is an agreement between two contracting parties, originally sealed with blood; a bond, or a law; a permanent religious dispensation.* These divine covenants or agreements were with Noah: The Noahic covenant; Abraham:the Abrahamic covenant; Moses: the Mosaic covenant, & with David: the Davidic covenant. These covenants served God's people as types and shadows and important reminders to the Israelites.
God interacts with His people today (the church) through the New Covenant, ratified not in the blood of bulls and goats, but in the blood of the Lamb of God, Jesus, who takes away the sins of the world. The old covenants prefigured the work of our Lord Jesus Christ in the work of redemption and reconciliation to God for whosoever will come to Him in humble faith. The liturgy that the priests served under was a system of worship; a copy or a shadow of the real one in heaven. But Jesus, our High Priest, has been given a ministry that is far superior to the old priesthood, and He is our lone mediator with the Father. The New Covenant (a better covenant) was not a type and shadow prefiguring Jesus as High Priest, High King, and the Savior of all mankind, but the substance and fulfillment of all that the old covenants implied.
As God used Israel in Old Testament Israel to serve as an elaborate type and shadow of His kingdom on earth, He now uses the New Testament church to spread the gospel to Jews and Gentiles alike and to worship Him in Spirit and truth. For this reason, St. Paul refers to the church as , the Israel of God. (Galatians 6:16)
The Church is a Gathering Place for Christians
". . . praising God and having favor with all the people. And the Lord added to the church daily those who were being saved." (Acts 2:47) - St. Luke's historical narrative of the Acts of the infant New Testament church.
When people of like interests discover one another, they usually arrange meetings to discuss their common interests. The church is no different. Christians share a common faith in Christ, a hope of eternal life, and a loving belief that we are our brother's (and sisters') keeper. It is in this faith, hope, and love that Christians assemble to worship the Lord who gave us all these virtues and to help one another in our struggles against the flesh, the world, and the devil.
The Church is One, Holy, Catholic, & Apostolic
The Church is One. St. Cyprian of Carthage, Epistle XXXIX, Letter to the People (ANF, Vol, V) There is one God, and Christ is one, and there is one Church, and one chair founded upon the rock by the word of the Lord. Another altar cannot be constituted nor a new priesthood be made, except the one altar and the one priesthood. Whosoever gathers elsewhere, scatters.
The Church is Holy. St. Augustine: The Holy Spirit the Soul of the Church (PL 38, Sermo 267; Sunday Sermons, Vol. III, 27-28) "... What the soul is to the body of man, the Holy Ghost is to the Body of Christ: which the Church is. What the soul does in all the members of one body, this the Holy Spirit does throughout the Church ..."
The Church is Catholic. St. Cyril of Jerusalem, Catechetical Lectures, XVIII (NPNF-II, Vol. VII) "The Faith which we rehearse contains in order the following, "AND IN ONE BAPTISM OF REPENTANCE FOR THE REMISSION OF SINS; AND IN ONE HOLY CATHOLIC CHURCH; AND IN THE RESURRECTION OF THE FLESH; AND IN ETERNAL LIFE"... It is called Catholic then because it extends over all the world, from one end of the earth to the other; and because it teaches universally and completely one and all the doctrines which ought to come to men's knowledge, concerning things both visible and invisible, heavenly and earthly; and because it brings into subjection to godliness the whole race of mankind, governors and governed, learned and unlearned; and because it universally treats and heals the whole class of sins, which are committed by soul or body, and possesses in itself every form of virtue which is named, both in deeds and words, and in every kind of spiritual gifts."
The Church is Apostolic. “Well, they preserving the tradition of the blessed doctrine derived directly from the holy apostles, Peter, James, John, and Paul, the sons receiving it from the father (but few were like the fathers), came by God’s will to us also to deposit those ancestral and apostolic seeds. And well I know that they will exult; I do not mean delighted with this tribute, but solely on account of the preservation of the truth, according as they delivered it. For such a sketch as this, will, I think, be agreeable to a soul desirous of preserving from loss the blessed tradition” (Miscellanies 1:1 [A.D. 208]). - CLEMENT OF ALEXANDRIA
“Again we write, again keeping to the apostolic traditions, we remind each other when we come together for prayer; and keeping the feast in common, with one mouth we truly give thanks to the Lord. Thus giving thanks unto him, and being followers of the saints, ‘we shall make our praise in the Lord all the day,’ as the psalmist says. So, when we rightly keep the feast, we shall be counted worthy of that joy which is in heaven” (Festal Letters 2:7 [A.D. 330]). “But you are blessed, who by faith are in the Church, dwell upon the foundations of the faith, and have full satisfaction, even the highest degree of faith which remains among you unshaken. For it has come down to you from apostolic tradition, and frequently accursed envy has wished to unsettle it, but has not been able” (ibid., 29). ATHANASIUS of Alexandria