The Earliest Witnesses
Now, it seems obvious to me that legends flourish on distance, translation and lack of contradiction from credible sources. So it would be a critical piece of the puzzle if we saw the story we have of Jesus becoming progressively more legendary as it spreads away from Jerusalem and as it is spread to people who are outside of the Aramaic/Hebrew-reading Semitic populations of the Levant.
Luckily, we have such a group. They are variously called the Ebionites or the Nazarenes. We know them largely through the lens of writers of an orthodox viewpoint who argued against their beliefs. However, their beliefs do in fact differ significantly from the proto-orthodox schools. It's instructive to see what the proto-orthodox report about these Christians of Palestine. So let's see what they have to say:
And the (Ebionaeans allege) that they themselves also, when in like manner they fulfil (the law), are able to become Christs; for they assert that our Lord Himself was a man in a like sense with all (the rest of the human family).
—Hippolytus of Rome, Refutation of All Heresies 7.22
The ancients quite properly called these men Ebionites, because they held poor and mean opinions concerning Christ. For they considered him a plain and common man, who was justified only because of his superior virtue, and who was the fruit of the intercourse of a man with Mary.
—Eusebius, Ecclesiastical History, Chp. 27
As to these translators it should be stated that Symmachus was an Ebionite. But the heresy of the Ebionites, as it is called, asserts that Christ was the son of Joseph and Mary, considering him a mere man
—Nicene Fathers, The Translator Symmachus, Chp. 17
For since they wish Jesus to be in reality a man, as I have said before, Christ came in him having descended in the form of a dove and was joined to him (as already we have found among other heresies also), and became the Christ from God above, but Jesus was born from the seed of man and woman.
They say that the Christ is the True Prophet and that the Christ is son of God by spiritual progress and a union which came to him by a lifting up from above; but they say that the prophets are prophets through their own intelligence and not from truth. Him alone they wish to be both prophet and man, and son of God and Christ, and mere man, as we have mentioned before, but because of excellence of life he came to be called the Son of God.
—Epiphanius of Salamis, Panarion 30.14.4-5 and 18.5-9
Vain also are the Ebionites, who do not receive by faith into their soul the union of God and man, but who remain in the old leaven of [the natural] birth, and who do not choose to understand that the Holy Ghost came upon Mary, and the power of the Most High did overshadow her: wherefore also what was generated is a holy thing, and the Son of the Most High God the Father of all, who effected the incarnation of this being, and showed forth a new [kind of] generation; that as by the former generation we inherited death, so by this new generation we might inherit life. Therefore do these men reject the commixture of the heavenly wine, and wish it to be water of the world only, not receiving God so as to have union with Him, but they remain in that Adam who had been conquered and was expelled from Paradise: not considering that as, at the beginning of our formation in Adam, that breath of life which proceeded from God, having been united to what had been fashioned, animated the man, and manifested him as a being endowed with reason; so also, in [the times of] the end, the Word of the Father and the Spirit of God, having become united with the ancient substance of Adam's formation, rendered man living and perfect, receptive of the perfect Father, in order that as in the natural [Adam] we all were dead, so in the spiritual we may all be made alive. For never at any time did Adam escape the harms of God, to whom the Father speaking, said, "Let Us make man in Our image, after Our likeness." And for this reason in the last times (fine), not by the will of the flesh, nor by the will of man, but by the good pleasure of the Father, His hands formed a living man, in order that Adam might be created [again] after the image and likeness of God.
—Ireneaus, Against Heresies 5.1.3
What shines clearly through this is that the doctrine of the trinity could not possibly have been the understanding of the Christians who supposedly lived the closest in time and space to the actual time of the legendary Jesus.
So the Christians who state that it's impossible to imagine people dying for their faith on the basis of a legend are failing to understand that the legend didn't exist for the Christians in Palestine. Specifically the Ebionites are said to have rejected the virgin birth, the divinity of Jesus, and his resurrection in a physical body, in addition to their acceptance of the Jewish laws as binding. They persisted in this belief for a very long time. Some authors have suggested that it was their ideas that Mohammed used in the Quran.
If early witnesses (eyewitnesses) are the most reliable, isn't the anomaly of the beliefs of the Ebionites one of the starkest problems with theories like Bauckham's?
There are more plausible explanations than that the early Jerusalem church was full of eyewitnesses. One possibility is that there are no eyewitnesses and the Greek gospels are fictions. This makes perfect sense of the situation and suggests that the Ebionites would naturally center in Palestine as they would have views consistent in the main with the other inhabitants of that region.
Another is that there was a man Jesus who was in no way regarded as a supernatural being by the people alive with him and may or may not have done any of the things reported in the Greek gospels but that he was remarkable in some way and legends began to adhere to his life story soon after he was gone. It's nearly impossible to be certain which of the above explanations is correct, but both have similar theological implications.
This map (which I got from Pagan Origins of the Christ Myth), shows the distribution of those heresies. This describes the situation as it existed prior to the conversion of the emperor, Constantine. The map makes great sense if you think that the Greek gospels are fictions since it allows for legendary development within geographical areas. Some stories that are at variance with what eventually became the dominant form of Christianity were clearly the mainstream view in large areas of the Christian world, and they were not uniform. This was the status quo prior to the empire burning all heretical books and plenty of heretics.
Thus we have the following facts:
1. The earliest Palestinian Christians did not believe in the divinity of Christ, his virgin birth, or his bodily resurrection.
2. They persisted in this belief for a very long time (even possibly until they Muslim conquest, when they may have converted to Islam).
3. The orthodox beliefs we consider central to Christianity today could not have been the beliefs that early Christian martyrs had (if there were early Christian martyrs), unless we postulate that God allowed heretics to dominate the Christian church in the most sacred place to it from the very beginning of the religion.