If you happen to have a debate with a Christian, the majority of quotes he will bring to defend the case of Jesus' death and resurrection as well as the doctrine of atonement, will inevitably come from some letter of a man called Saul of Tarsus, better known as Paul. You will hear quotes from Galatians, Colossians, Timothy, Thessalonians, etc. - letters Paul wrote to different churches and persons in his time.
Paul never met Jesus. He was not one of the original disciples. In his writings he indicates that he had a "vision" of Jesus on the road to Damascus. Allegedly Jesus then chose him to proclaim his Gospel. The "correct one", as Paul states in his writings, speaking against those "who preach another gospel", which would indicate that there were those who indeed preached something different from Paul. Apparently some of these were the original disciples themselves, who were strongly opposed to Paul. Why was this the case? Well, for one, they saw Paul as a "false disciple" who preached things opposite to what Jesus preached. Unfortunately, we do not have any writings from the disciples anymore, only those of Paul, and some influenced by Paul. It is clear that Paul was trying to portrait his relation to the disciples as "good", but even in some writings we have today in the New Testament we see the friction that reigned between both parties.
What was it then that Paul preached and was different from what Jesus or the disciples preached? As a main point, Paul was the first person to preach the doctrine of resurrection, that is, that we are now saved "in Jesus", because Jesus gave his life to atone for the sins of the humanity.
How about a comparison with the Gospels? We cannot really contrast the information because all four Gospels in the NT were written after Paul, and after this doctrine was well established and had prevaled over other doctrines, mainly those preached by the real disciples. So, the writings in the Gospels are strongly influenced by the teachings of Paul.
It is this doctrine of Paul that will later create many difficulties for the Gospel writers and other followers of Jesus - because the writers now had the task to attempt to rework many stories about Jesus to be in accordance with pauline teachings. We see this throughout the gospels - and in some cases we see apparent contradictions between the different books as well as in the same book, because the authors used sayings of Jesus which had survived in the oral (and maybe written) tradition in their books, together with the teachings of Paul - thus creating confusing theology where we have a Jesus saying that he was only sent to the house of Israel, and later the same Jesus commanding his "disciples" to go out and preach his Gospel to the whole world.
The latter part naturally came as an influence from Paul, who saw his role as being the one to preach the Gospel to the gentiles - the authors thus had to construct a story in their Gospels, presenting Jesus as commanding this. One might ask why then did they not take out the first quote where Jesus says 'he is only sent to the house of Israel'? It is reasonably possible that these sayings of Jesus were publicly known, passed on by generations, and one could not simply get rid of them like that.
While this particular issue might not be that grave, from a theological standpoint - it is the other major parts of the pauline doctrine that had a tremendous negative impact on the later followers of Jesus. Paul was the first to claim that Jesus died as a curse from God - he was basing this in the Old Testament where it says that "whosoever dies hanging" is cursed from God (Deuteronomy 21:22-23). But now, having claimed this, Paul was left with a difficulty, since how could Jesus, an agent of God be cursed? This is the part where Paul got creative and introduced the doctrine of resurrection - his solution to the cross conundrum. Since God resurrected Jesus, then this means he was not cursed. Jesus beat death, in a manner of speaking, thus anyone who accepts Jesus as his Lord, can beat death too.
But that has created the apologetic burden for the Gospel writers to actually prove that Jesus was resurrected.
The whole later parts of the Gospels, dealing with the resurrection are directly based on this pauline doctrine - and the Gospel writers constructed their stories to reflect this belief. This is absolutely visible if we take the chronological order of the Gospels. Mark, being the first written Gospel, has almost nothing on resurrection. After him, Matthew and Luke, who based their writings on those of Mark, developed the story further. And as the years would pass by, and the pauline doctrine was getting stronger and stronger - so was the reflection of the doctrine more prononunced - culminating in the Gospel of John, which was written almost 60 years after Jesus - and where the personality of Jesus is developed fully as preached by Paul - of him being our "Lord" and the agent through whom God made everything.
This development is very visible in the Gospels. As the doctrine of Paul was becoming more and more prevalent, so are the writings of the gospel authors. If we have Jesus in Mark healing a few people in a village, in Matthew and Luke he heals the whole village. And whereas in Mark he feeds a group of people, in the later Gospels he feeds thousands of them. And bear in mind, these are the same stories, but developed more and more, the later the Gospel.
Finally, in John, Jesus is no more weak, never gets angry, doesn't pray to God to save him, and boldly uses claims about himself which were spoken by God in the Old Testament.
Why would Saul of Tarsus do all of this?
The question is hanging in the room. Why? What would compel Saul of Tarsus to do all this? To understand that, we shall reflect on his name first. Why is he called Paul and Saul? What's that all about?
Well, it just so happens that Paul was actually known as Saul of Tarsus for many years. A hellenistic (greek speaking) Jew, who was known for his ruthlessness as a torturer of the followers of Christ. He was in fact a mercenary, a head hunter - hunting followers of Jesus wherever he could find them.
We now have two possibilities to explain his "sudden change of heart". The third one - him being a real disciple of Jesus, commanded by Jesus to do what he did - falls short because of all the evidence provided above.
The first of the two possibilities is that he suffered from some kind of an inferiority complex - thus wishing to make himself "special" and a leader. He would do anything to achieve this, and as he himself pointed out, he would not hesitate to lie and deceive in order to preach "his" Gospel to the gentiles (2 Corinthians 12:16, I Corinthians 9:19-22).
The second possibility, is that maybe he saw an opportunity to finish what he could not accomplish as hunter of the followers of Jesus. Perhaps he saw that the movement of the believers in Jesus was getting stronger, and that also "stronger measures" were needed to counteract this. What better way than to do it from within?
Be that as it may, the fact is his preaching of this "new" Gospel became the mainstream belief of the Christians.
What happened to the teachings of the original disciples?
Christian apologetists of today often argue that if the disciples of Jesus preached some other form of Christianity then why do we not have any of their teachings in any manuscripts? They so often need to be reminded that in fact we do have manuscripts which point to many other forms of Christianity, and I am not even thinking of the gnostic Gospels right now, although they should be taken into consideration as well.
Even in the Gospels of today which were canonized some three centuries after Jesus, biblical scholars have found remnants of what must have been some of the original teachings of Jesus and his disciples. The most evident example are the three synoptic Gospels, that according to Mark, Matthew and the Gospel according to Luke. It has been known for some time now that Matthew and Luke used Mark as their source - thus the overlapping stories, but scholars have also noticed that both Matthew and Luke have also incidents in their Gospels which one does not find in Mark, leading to the conclusion that they must have had another source, named "Q" (from the german word for source - "Quelle").
What is most interesting though, is that in the "Q" sayings, called as such because they are only comprised of sayings of Jesus, there is no mention of any biographical material about Jesus which we normally find in the Gospels. The Q sayings seem to be part of the original "Gospel" of Jesus, that is, the book that was revealed to Jesus from God. There is nothing in the Q about the "crucifixion" of Jesus or his "death" or "resurrection". There is no mention of empty tombs and atonement for mankind. In reading Q, one will find a remarkable similarity of this "Gospel" to another revelation from God - the Qur'an, revealed to the last Prophet, Muhammad (saws).
It is then reasonable to assume that the disciples of Jesus preached exactly this, as "the Gospel" - leading us to some understanding of why there was friction between them and Paul - who preached something completely different and utterly alien to what Jesus actually preached.
There is also knowledge about different sects that existed up to the third century which did not believe in Jesus as God or part of the trinity, and regarded him rightly so, as a prophet of God. Among these sects were also the Nazarenes and Ebionites. Later in the third and fourth century, the followers of the christian presbyter Arius still maintained the Divinity of God the Father over Jesus the Son. For this they were persecuted by the mainstream church after the First Council of Nicea which basically created the doctrine of Trinity. Emperor Theodosius I effectively wiped out Arianism once and for all among the elites of the Eastern Empire through a combination of imperial decree, persecution, and the calling of the Second Ecumenical Council in 381, which condemned Arius anew while reaffirming and expanding the Nicene Creed. This generally ended the influence of Arianism among the non-Germanic peoples of the Roman Empire and Pauline Christianity became the mainstream religion of the Empire.
It is more than evident that Paul took the teachings of Jesus and his disciples and substituted them with his own. He twisted and changed the monotheistic doctrine of Jesus and turned it into a mix of judaic teachings with helenistic pagan ingredients. Through this, he "created" another Jesus - one that is not merely a prophet anymore, but a divine figure, a Lord and a Saviour.
He adapted his doctrine even more to the pagan beliefs in order to make it more acceptable to the gentile pagans he was preaching to. So, if some pagans in the roman empire liked to eat pork very much, he told them "oh, that is no problem, you can continue to eat pork as long as you accept Jesus as your Saviour!", and if some other politheistic helens cherished their old held beliefs of ancient gods who had sons and daughters - then Paul adapted the doctrine and told them that Jesus is also a Lord (theos, basically a god) and the Son of God. This way it was far easier for the gentiles to accept Paul's doctrine.
Paul succeeded in creating a whole new religion which was not Christianity anymore, but rather Paulianism and which was not the religion of Christ anymore but rather the religion about Christ. From that point on the trinitarian Christians followed Paul instead of Jesus.
Therefore those 2+ billion people calling them selves Christians today are in fact Paulians.