...and those were WELL DOCUMENTED and the Church didn't touch them with a ten-foot pole. Scriptures being tampered with IS a contemporaneous claim that doesn't consider the history of Church and Her Sacred Tradition.
^^ This.
It is true that the movement that eventually was to become mainstream Christianity or "the Church" was
influenced by Gnosticism, though not in the fashion such conspiracy theorists think:
To every action, there was a reaction, and these competing factions reacted to each other by tackling the position of the other side. As such, the gospel of John was influenced by Gnosticism, but it is emphatically
not a Gnostic gospel, but aimed at refuting some of the traditional Gnostic positions.
For example, the divinity and/or humanity of Jesus had become a major point of contention by the end of the 1st century CE. Gnostic sects tended to deny that Jesus was a human being, holding that matter itself was fundamentally tainted and as such unfit as a vessel for the divine. To them, Jesus was more like a hologram, a being of spirit taking a human form without actually descending into the material universe of the Demiurge. Judaic sects, on the other hand, denied Jesus's divinity, considering him a messiah in the classical sense of the word: an anointed, inspired leader rather than an avatar.
The sect that was to become the Church we're familiar with today chose a third option: Jesus was BOTH fully God AND fully human. Enter the gospel of John...
Joh19: 34 Instead, one of the soldiers pierced Jesus' side with a spear, bringing a sudden flow of blood and water. 35 The man who saw it has given testimony, and his testimony is true. He knows that he tells the truth, and he testifies so that you also may believe.
Few contemporary readers understand what this is actually about. What's so important about this business that it must be emphasized? Clearly, the original audience would have known what this was about, or else it would not have made too much sense to point it out. And here it is:
See, in the ancient world, people believed that there was no blood in the veins of deities, but merely water/ichor. In Jesus's veins, however, there was supposedly BOTH, for he was both fully man and fully God. So, the whole "testimony" is a refutation of both the Judaizers and the Gnostics. Plus, it emphasizes that Jesus was really dead, as the wound is described in a fashion that ascertains its fatality, thus tackling another point of contention between the competing Christian sects.
In the milliennia that passed inbetween, the Church has pretty much forgotten about this original context, too, and many commentaries regard this passage as somewhat of a mystery - yet it's not. Not in the light of what we know about late antiquity and the heretical sects.