Its a mess. Because all these teachings were made up by various people over hundreds of years.
“The oldest surviving complete text of the New Testament is the Codex Sinaiticus, dating back to the middle of the fourth century. The oldest fragments, the Bodmer and Beatty Papyri and Papyrus 52, date back to the second century but only contain bits of the Gospel of John. All of these texts are Greek.
Jesus's native tongue was Aramaic, and even if he knew Greek, he certainly did not speak it to his apostles, many of whom were uneducated fishermen. Without any surviving Aramaic texts, the actual words of christ are lost forever, mired in a sea of subjective translation by ancient scribes.
There are three hundred years between the composition of a text and our surviving copies. In a world without a printing press, texts would often undergo drastic evolution through centuries of handwritten duplication.
Our four canonical gospels did not begin their lives as the gospels of "Matthew," "Mark," "Luke" and "John." Different groups of early Christians maintained their own oral traditions of Jesus's wisdom, as writing was a specialized skill and not every fellowship enjoyed the services of a scribe. When written accounts of Jesus's teachings began to circulate (i.e., the theoretical "sayings" gospel Q and the Semeia or Signs source), the independent groups WOULD SUPPLEMENT THEM WITH THEIR OWN TRADITIONS about the savior, each believing their own versions to be "the Gospel." Eventually, as these expanded writings spread through other communities, some versions were viewed as having more authority than others. It was not until the pronouncement of Bishop Irenus (185 C.E.) that Christians began to accept only the four familiar gospels as authoritative, and to refer to them by their modern titles.
The rest of the canon was much slower to develop. For the next two centuries, the four gospels would be coupled with a myriad of different letters, epistles, stories and apocalypses, according to what a particular congregation JUDGED AS RELEVANT TO THEIR UNDERSTANDING of Jesus Christ and his message. Catholicism was only one of the dozens of "denominations" within the early church—Gnosticism was prevalent throughout Egypt, Montanism in Asia Minor, Marcionism in Syria.
Eventually, the Catholic church was adopted as the state religion of the Roman Empire, and all other systems of belief were branded as heresies. Following the Epistle of Athanasius in 367 C.E., the Church finally reached agreement upon which writings were truly authentic and representative of apostolic tradition, thus forming what we know today as the canonical New Testament. Although factions of the Church continued to debate the merits of various books for centuries, and many even used other writings in their liturgy, most uncanonical writings were ordered to be destroyed. In many cases, possession of heretical literature was punishable by death.”