True Doctrines Or Heresies? I am a Protestant. I have been trying to trace the church from the time of Jesus to the current day. I have a question.   

In the first half a century I see great debates in the defense and development of true doctrine. During this time central truths were developed such as the doctrine of the Trinity and the Incarnation. At least 24 major heresies were withstood, and the New Testament was canonized. If any of this would have failed to happen, Christianity, as we know it today, would not look the same.

Here is my question.   

Many of the doctrines of the early church, such as tradition, liturgy, icons and so on were all well established by the third and fourth centuries. They have been rejected by the Protestant faith as false teachings. But these doctrines were developed close to the source where the water is usually the purest. They were developed by men that gave their lives for what they believed and taught. If they were heretical, would the streets have not run with blood, yet it seems there was unity on most doctrines?
In my understanding, Protestantism takes the view that "if not in the [Protestant] Bible, it's human doctrine" (view more formally called sola scriptura). And if human doctrine, it is therefore fallible and should be judged according to their merits and consistency with the rest of the biblical teaching. This is the origin of many Protestant rejections of Catholic and Orthodox doctrines.

Now, you are asking for these other practices that are not in the [Protestant] Bible. Are they true doctrines? Well, this is surely a matter of debate. A Protestant apology of the adoption of some of such doctrines is in this answer. Ultimately, the key issue is to whether there is authority in what the Catholic and Orthodox churches call Tradition, i.e. the rules of faith and practice that were (allegedly) transmitted by the Apostles to their own disciples and churches, and which are not explicitly in the Bible. It is pointless to deny the existence of these traditions. St. Paul himself states:


Corinthians 11:2



  I praise you for remembering me in everything and for holding to the traditions just as I passed them on to you.



2 Thessalonians 2:15



  So then, brothers and sisters, stand firm and hold fast to the teachings we passed on to you, whether by word of mouth or by letter.


The debate is about what exactly does this tradition include and to which extent it is necessary for salvation.

It should be remembered here that not even within Protestantism there is a unified voice about what doctrines and practices are valid or not. For instance, an issue as basic as the Eucharist is very contentious (e.g. see here). Not all hold Sunday as the observance day either. And so on. In effect, here the Catholic Church states that actually, the Bible alone is not enough to define all necessary doctrines of faith and practice. Hence its recourse on Tradition for things like the seven sacraments, and so on. 

Finally, notice also that the canon of the Bible is different among denominations. Thus, for instance, things like the purgatory are justified in Catholicism based, among other things, in 2 Maccabees, which is not part of the Protestant canon. Thus, even without recourse to tradition, mere canon differences give rise to doctrinal differences among denominations.