How to know what is true
The authority for Christian teaching and doctrine is the apostles, apostolic teaching. Yes, the apostles wrote the New Testament and it is, therefore, apostolic. (But we must weed out a few errors in their teaching.)
The following factors are the necessary ingredients in defining the Christian faith...
The Bible is not the one and only source of authority (Sola Scriptura)...
We must interpret the Bible in light of known true doctrine. Even the Protestant reformers (revolutionaries) did this, but they won't admit it. (No, they did not derive their doctrinal system from scratch from the Bible as they claim, rather, they started with Catholic teaching and only discarded some of it.)
The Word of God manifests in many ways. Some examples...
The source of Divine Revelation is God himself; he revealed his Living Word to the apostles and prophets directly, not through writing. Later these apostles and prophets shared these received revelations both verbally and by writing them down. But the Word of God is the authority, not the medium of communication of the message (for example, the writings).
Certainly the Bible contains the Word of God and is authoritative, but the Word of God takes other forms also. For example, the Word of God manifested in the council of Nicea when the bishops formulated the Nicene Creed in response to the Arian heresy.
The apostles were the first leaders of the Christian church. They had the authority (1) to teach correct doctrine, and (2) to interpret scripture.
We have also a more sure word of prophecy; whereunto ye do well that ye take heed, as unto a light that shineth in a dark place, until the day dawn, and the day star arise in your hearts. (2 Peter 1:19)
The apostles possessed the reliable prophetic message and this message is the important thing whether communicated via scripture or tradition.
I have written unto you, fathers, because ye have known him that is from the beginning. I have written unto you, young men, because ye are strong, and the word of God abideth in you, and ye have overcome the wicked one. (1 John 2:14)
The Protestant Reformers opposed what they called traditions of men but we must distinguish between sacred traditions and human traditions. Sacred traditions are those truths of the Christian faith bringing redemption.
We must allow for human traditions that do not violate the Christian faith. Examples of these...
We are to stand firm in the traditions of the apostles, including oral traditions. Certainly manmade traditions have no binding spiritual authority, this in contrast to the traditions and teachings passed-down from the apostles which are authoritative.
Now we command you, brethren, in the name of our Lord Jesus Christ, that ye withdraw yourselves from every brother that walketh disorderly, and not after the tradition which he received of us. (2 Thessalonians 3:6)
Apostolic tradition is the measure of true Christian faith.
My conclusion about the true story of what really happened...
With so many voices clamoring to be heard, claiming that truth is such and such, how are we to know who to believe? Some possibilities...
My view on how to get at the truth: where to start, what to assume, the trail of clues we must follow...
We must start with two facts: (1) we are conscious entities, and (2) animistic and pagan religions were practiced nearly universally from very early in human history.
Therefore, we can rule out atheism. However, we must consider the conclusions of modern science (including evolution, psychology, behavioral biology) when addressing aspects of the physical realm. And we must consider the conclusions of modern archaeology with its use of modern science in dating artifacts.
Evil exists outside of God and is a living spiritual entity; a spiritual being chooses to operate in free will outside of God's nature.
God would want to interact with those cognitive creatures (humans) he created. God would want us to know the truth about his nature and plan and purposes. God would allow us to stumble through our attempt to discover truth using our free will. Therefore, over time, he would reveal more accurate information about himself.
We should expect God to reveal monotheism to humanity at some point. Probably would single out a person (Adam) to reveal this to, then pass on to patriarchs (Noah, Abraham) of a tribe through multiple generations (the Israelites), improving in accuracy over time.
Which other religions match this criteria? We can rule out eastern philosophy with its reincarnation and polytheism — they don't match at all. Only Judaism, Christianity, and Islam qualify as monotheistic. We can rule out the later of these with its conquering of much of the world by the sword.
We should expect God to take on human form and human nature at some point to conquer evil. Judaism is missing this aspect, seeming incomplete somehow.
How would the incarnate deity comport himself? He would teach monotheism and truth. He would collect disciples who would become apostles and would write down the teachings. The incarnate deity would conquer evil, providing for redemption. Jesus, of course, is this incarnate deity and he conquered evil though his death and resurrection.
We should expect the true religion to appear after the age of philosophy. Philosophy appeared with only one people in the history of humanity, with the Greeks. Philosophy provides the foundation for knowing about beliefs, ideas, truth, knowledge, and the nature of reality including the foundations of the scientific method for determining knowledge of the physical world.
We should emphasize the importance of using writing to pass on these teachings. Only in the Roman Empire could writings be passed on to a large audience with copies stored at many places in the empire. And the Roman Empire had peace, good roads, and freedom of movement. Certainly writing could be maintained from the time of the first kings of Israel, David and Solomon, with the temple having priests and scribes.
There should be a canon of scripture. Rules are needed to determine which writings to include. Only the writings of an apostle should be included since they are the authorities, having been taught by Jesus.
We should have an early creed having the essential doctrines of the faith. Of course, Christianity has this in the Nicene creed which is very harmonious with the New Testament.
In judging the truth of claims made by anyone, these points above should provide the criteria for judging.
Thus, we should reject the following...
But we should also reject...