The Fundamental Question
The fundamental question we must answer is "who do we trust?" When it comes to spiritual truths and eternal salvation we require divine revelation to guide us. But what form does this take? Some possibilities ...
From the earliest days after the founding of the church, people have wandered away from the truth of Christ and lapsed into heresy. Even the apostle Peter who should have known better lapsed under pressure from the Jews and had to be rebuked by Paul.
Why would Jesus create a church which seemingly could not remain united? And why are there literally thousands of denominations and other Christian communities each with differing beliefs?
Perhaps Jesus knew that in order for the church to survive it would need to take various forms, just as God confused the languages of man (Genesis 11:9) and we now have many languages. But in the early days of the church near the time of the apostles the foundational doctrines were established.
The first answer is that we must trust Jesus. Read why this is so.
What was the early church really like? There have been several stages ...
The only guides people had were: (1) teachings of the apostles, (2) teachings of Jesus which some people had heard first-hand.
These writings were not available to everyone. They were read during the church services when available. Personal bible study was not the norm.
But there were many other writings which were also read during the church services, books and letters which were not included in the New Testament.
The apostles had validated and ordained the next generation of teachers. Certainly they expected these new teachers to be a teaching authority for the people.
A difficult time for Christians. Which teacher has the true teaching? Some of the heretics were ordained bishops. They all used the Bible to support their views.
These creeds provided (and still provide) guidance for Christians at large. The orthodox bishops struggled with the heretical bishops and truth finally prevailed — the Trinity, the deity of Christ, etc.
Here is where the seeds of disunity begin. In the east the church was part of the state. In the west the popes began asserting their primacy (but the Orthodox Church believes that the see of Rome possessed true doctrine until about 800 A.D.). Emphasis on the hierarchy of the clergy which resulted in the disenfranchisement of the laity. Christendom developed in which the church hierarchy competes with state authority.
Certainly for the first 800 years of church history the Patriarchate of Rome (the pope) defended orthodoxy in the face of severe heresies. Even the Orthodox Church admits that it was the pope that guided the church into all truth during this time.
Once the great schism of 1054 A.D. occurred between east and west there was no longer the possibility of institutional unity in the church. Both east and west had validly-ordained bishops and a valid Eucharist but they could no longer meet in an ecumenical council.
John Henry Cardinal Newman discusses the topic of whether we can trust the early church fathers to determine doctrine. He notes that certain key doctrines such as the Trinity and original sin can not be supported merely from the early church fathers. Thus, we require a teaching authority. The Catholic Church claims to have the true teaching authority (the magisterium) but some of the Catholic teachings of the last 1,000 years contradict the teachings of the Orthodox Churches.
My conclusions ...
Based on the above points, the reliable guides for truth are ...
In the early chapters of the book of Revelation the apostle John comments on the seven parishes of his diocese; we discover that the majority of these are clearly dysfunctional. Catholics could use this to support their view that it is bishops (Catholic bishops) who are the teaching authority, and that their varying degrees of "badness" throughout history and into the present does not nullify this. (But I've never heard of anyone using this passage this way.)
When I was Catholic I found it very sad that: (1) the pope can't ensure that every bishop is an on-fire-for-the-Lord, evangelical Christian, and (2) bishops can't ensure that their priests are orthodox and that they follow the teachings of the Catholic Church. For example, my (former) bishop didn't even seem to be a Christian; he never talked about Jesus or salvation or repentance. Rather, he seemed more concerned with the administration of his diocese, the maintenance and promotion of his cathedral, and encouraging people to embrace their Catholic traditions (but excluding the gospel from the list of traditions).
The Catholic Church teaches that it is the bishops (in unity with the pope) who provide true teaching. But why would the Holy Spirit use "bad" bishops and "bad" popes in this way? Also, in the early church it seems that many of the most inspirational and orthodox church fathers were not bishops at all.
The first ecumenical (all-church) council in Nicea developed a creed containing the doctrine of the deity of Christ; this creed is obviously orthodox. Yet many of the bishops present were heretics and the presence of the pope was minimal. During the council there were fights and all kinds of less than saintly behavior by those present. The emperor Constantine called together the council with the goal of bringing unity to the church and his presence ensured that this goal was accomplished. How could a council of "bad" bishops dominated by an emperor be used by the Holy Spirit?
Another example is the Old Testament nation of Israel. Moses, a murderer, founded this nation. David, another murderer, was its foremost king. God finally had to send prophets who were persecuted by the nations leaders. And yet this nation of Israel provided the foundation for the gospel and the Christian church.
Based on these examples from history, the Catholic Church sees no problem with its doctrine of the teaching magisterium(authority) which holds that true teaching and doctrine can only come from bishops who have been ordained with apostolic succession and who are in unity with the pope.
Throughout the history of the church we see many examples of bishops engaged in bad leadership; of bishops who can't maintain discipline of their parishes; and some of the priests are heretics or are immoral.
Even the apostle John seemed to be a bad church leader in this regard. His parishes had all kinds of serious problems. Presumably, the ordained leaders of his parishes were involved in these abuses or were at least unwilling or unable to stop it. Therefore, bad church leadership and morals by church leaders does not prevent these same leaders from being God's chosen teaching authority.
However, using John the apostle to frame this argument has some flaws. John was holy; the "bad" bishops are not. John taught orthodoxy; the "bad" bishops do not. John was chosen by Christ; the validity of the ordination of "bad" bishops is in question.
I provide examples to demonstrate that apostolic succession has been broken on many occasions. Also, throughout history there are plenty of example of bishops who were ordained using dubious ordination procedures.
In conclusion: Just because the apostle John's parishes were in disarray doesn't mean that we should discount the bad behavior and improper ordination of Catholic bishops in determining whether or not their teachings are true and trustworthy. The apostles are our source of authentic truth; the bishops are not (but many bishops taught and continue to teach the true gospel handed-on from the apostles). In addition, the clear teachings of the Bible should be emphasized.
I should mention that I believe that many Catholic doctrines are correct including purgatory, the Marian doctrines, the Eucharist, salvation by a combination of faith and works, the Saints, the validity of the use of icons and statues, the mass as the proper worship of the church, and many others.