In the Nicene Creed, which is accepted by most Christians, the Christian Church is described as being "one, holy, catholic, and apostolic." These are known as the four marks of the Church. The notions of holiness and catholicity are not much in dispute. The mark of holiness may be defined as the possession, and dissemination of the sublime, holy, Christ-centered moral code of Christianity (as best exemplified by saints or otherwise great, godly figures).
All parties - while disagreeing on many particulars - concur that this is a central function of the Church. Catholicity simply means universal. Here Protestants and Catholics disagree only on the nature of that Church which is to be considered universal and all-encompassing.
This brings us to the oneness and apostolicity of the Church, where the disagreements are great indeed. Most Protestants (especially evangelicals) see unity and oneness subsisting primarily or solely in the inner, invisible, spiritual unity of those who are in fact in Christ by virtue of being justified, or born again, or regenerated (with or without baptism, depending on denomination). For them, the church consists of the Spirit-filled, predestined elect, who will persevere and are saved, now and in eternity.
The Catholic Church has always proclaimed this unifying characteristic also, under the broad and rich concept of the Mystical Church (under which it acknowledges Protestantism), yet it doesn't pit the Mystical Church against the institutional, or visible Church, as most evangelicals do. For Catholics, then, the issue of oneness is substantially related to organizational and practical aspects of ecclesiology. Catholics believe that the Church is both organism and organization, not merely the former. The Mystical and visible "churches" are like two circles which largely intersect, but which are not synonymous. They exist together - somewhat paradoxically and with tension - until the "end of the age."
Catholics appeal to the hierarchical, or episcopal (that is, under the jurisdiction of bishops) nature of Church government. Furthermore, Catholics maintain that this form is divinely-instituted and biblical, therefore not optional or of secondary theological importance.
Catholics believe that bishops are - by the intention of Jesus Christ - the successors of the Apostles (the concept of apostolic succession). This is the methodology whereby the Catholic Church traces itself back historically in an unbroken succession to the Apostles and the early Church. Catholicism thus greatly emphasizes both historical and doctrinal continuity, whereas evangelical Protestants are more concerned with maintaining the passion and intense commitment and zeal of the Apostles and early Christians, and are less interested in governmental forms or doctrines which are now regarded as Catholic "distinctives." They tend to see clearly in the Bible and early Church those doctrines with which they agree, but overlook those which are more in accordance with Catholicism, such as the episcopacy, purgatory, and apostolicity.
One of the undeniable aspects of unity and oneness in the Bible is the constant warning (especially in the writings of St. Paul) against (and prohibition of) divisions, schism, and sectarianism, either by command, or by counter-example (Matthew 12:25, 16:18, John 10:16, 17:20-23, Acts 4:32, Romans 13:13, 16:17, 1 Corinthians 1:10-13, 3:3-4, 10:17, 11:18-19, 12:12-27, 14:33, 2 Corinthians 12:20, Galatians 5:19-21, Ephesians 4:3-6, Philippians 1:27, 2:2-3, 1 Timothy 6:3-5, Titus 3:9-10, James 3:16, 2 Peter 2:1). This is clearly no trifling matter. Our Lord even makes unity a means by which the world might believe that the Father sent the Son (John 17:21,23), and prays that it will be as profound as the unity of the Trinity itself (John 17:21-22). St. Paul makes stirring up division a grounds for virtual exclusion from the Christian community (Romans 16:17), and says that divisions (in effect) divide Christ (1 Corinthians 1:13). This has always been one of the strengths of the Catholic position over against Protestantism, and Protestants are themselves increasingly alarmed over what they consider to be a scandalous concurrence between denominationalism and sectarianism, which all agree is condemned in Scripture.
Jesus chose Judas as His disciple, even though He knew the future, and he was truly an Apostle (Matthew 10:1,4, Mark 3:14, John 6:70-71, Acts 1:17). Likewise, St. Paul, in addressing elders (Acts 20:17) states that the Holy Spirit Himself has made them bishops (RSV, guardians, Greek, episkopos - Acts 20:28), yet from among these very same men, heretics and schismatics would arise (Acts 20:30). He echoes this thought in the parable-like verse 2 Timothy 2:20 (see also 2:15-19).
Protestants often cite Jesus' analogy of sheep and shepherd (John 10:1-16; cf. 2 Timothy 2:19, 1 John 2:19), who know each other (10:14), as evidence that the Church consists of the elect only. Yet the analogy breaks down when we find that Scripture also applies the term sheep to the unsaved reprobate (Psalm 74:1), the straying (Psalm 119:176), Israel as a nation (Ezekiel 34:2-3,13,23,30), and, indeed, all men (Isaiah 53:6).
Other passages which presuppose a visible, identifiable, "concrete" Church include Matthew 18:15-17, in which believers are exhorted by our Lord to take errant and obstinate brothers to the church, which will then determine the appropriate verdict. It would be contrary to the tenor of the New Testament if this were a reference to a local church alone - even apart from the utterly impractical consequences of such a scenario (where the sinner would simply attend another denomination and move on with his life, as is tragically all too often the case today).
And St. Paul, in 1 Timothy 3:15, describes the "church of the living God" as "the pillar and bulwark of the truth." This statement is similarly almost nonsensical in the context of competing and often contradictory denominations. Where would a sincere, uninformed, unsophisticated religious seeker go to find this certain truth? Only within the sphere of a serious attempt at actual visible oneness of doctrine can this verse attain any pragmatic possibility.
It is also incorrect to regard St. Paul as some kind of spiritual "lone ranger," on his own with no particular ecclesiastical allegiance, since he was commissioned by Jesus Himself as an Apostle. In his very conversion experience, Jesus informed Paul that he would be told what to do (Acts 9:6; cf. 9:17). He went to see St. Peter in Jerusalem for fifteen days in order to be confirmed in his calling (Galatians 1:18), and fourteen years later was commissioned by Peter, James, and John (Galatians 2:1-2,9). He was also sent out by the Church at Antioch (Acts 13:1-4), which was in contact with the Church at Jerusalem (Acts 11:19-27). Later on, Paul reported back to Antioch (Acts 14:26-28).
The New Testament refers basically to three types of permanent offices in the Church (Apostles and Prophets were to cease): bishops (episkopos), elders (presbyteros, from which are derived Presbyterian and priest), and deacons (diakonos). Bishops are mentioned in Acts 1:20, 20:28, Philippians 1:1, 1 Timothy 3:1-2, Titus 1:7, and 1 Peter 2:25. Presbyteros (usually elder) appears in passages such as Acts 15:2-6, 21:18, Hebrews 11:2, 1 Peter 5:1, and 1 Timothy 5:17.
Protestants view these leaders as analogous to current-day pastors, while Catholics regard them as priests. Deacons (often, minister in English translations) are mentioned in the same fashion as Christian elders with similar frequency (for example, 1 Corinthians 3:5, Philippians 1:1, 1 Thessalonians 3:2, 1 Timothy 3:8-13).
The primary controversy among Christians has to do with the nature and functions of both bishops and elders (deacons have largely the same duties among both Protestants and Catholics).
Catholics contend that the elders/presbyters in Scripture carry out all the functions of the Catholic priest:
1) Sent and Commissioned by Jesus (the notion of being called): Mark 6:7, John 15:5, 20:21, Romans 10:15, 2 Corinthians 5:20.
2) Representatives of Jesus: Luke 10:16, John 13:20.
3) Authority to "Bind" and "Loose" (Penance and Absolution): Matthew 18:18 (compare Matthew 16:19).
4) Power to Forgive Sins in Jesus' Name: Luke 24:47, John 20:21-23, 2 Corinthians 2:5-11, James 5:15.
5) Authority to Administer Penance: Acts 5:2-11, 1 Corinthians 5:3-13, 2 Corinthians 5:18, 1 Timothy 1:18-20, Titus 3:10.
6) Power to Conduct the Eucharist: Luke 22:19, Acts 2:42 (compare Luke 24:35, Acts 2:46, 20:7, 1 Corinthians 10:16).
7) Dispense Sacraments: 1 Corinthians 4:1, James 5:13-15.
8) Perform Baptisms: Matthew 28:19, Acts 2:38,41.
9) Ordained: Acts 14:23, 1 Timothy 4:14, 5:23.
10) Pastors (Shepherds): Acts 20:17,28, Ephesians 4:11, 1 Peter 5:1-4.
11) Preach and Teach: 1 Timothy 3:1-2, 5:17.
12) Evangelize: Matthew 16:15, 28:19-20, Mark 3:14, Luke 9:2,6, 24:47, Acts 1:8.
13) Heal: Matthew 10:1, Luke 9:1-2,6.
14) Cast Out Demons: Matthew 10:1, Mark 3:15, Luke 9:1.
15) Hear Confessions: Acts 19:18 (compare Matthew 3:6, Mark 1:5, James 5:16, 1 John 1:8-9; presupposed in John 20:23).
16) Celibacy for Those Called to it: Matthew 19:12, 1 Corinthians 7:7-9,20,25-38 (especially 7:35).
17) Enjoy Christ's Perpetual Presence and Assistance in a Special Way: Matthew 28:20.
Protestants - following Luther - cite 1 Peter 2:5,9 (see also Revelation 1:6) in order to prove that all Christians are priests. But this doesn't exclude a specially-ordained, sacramental priesthood, since St. Peter was reflecting the language of Exodus 19:6, where the Jews were described in this fashion. Since the Jews had a separate Levitical priesthood, by analogy 1 Peter 2:9 cannot logi