Answer:

It is true that there have been a small number of interpolations into the New Testament text.  Just for your information, Luke 22:43-44 , 1 John 5:7-8, and John 7:53-8:11 are all certainly interpolations.  Mark 16:9-20 is probably an interpolation, and  Luke 22:19-20 and 1 Cor 14:34-35 are most likely NOT interpolations.  Almost all ancient manuscripts have the last two in them, none of the earliest include the first three and only a few of the earliest ones include the Mark 16 passage.  So, what you have heard is true to a large extent, but perhaps some of what you have read is exaggerated.

In any case, the cause of the interpolations probably vary.  In the case of John 7:53-8:11 it is most likely that this is a genuine scene from the life and ministry of Jesus, the early church wanted to include it, but did not know where to include it.  The one who inserted it into John where it is now almost certainly believed it is genuine but did not know where it belonged.  In the case of 1 John 5:7-8, this is a much later attempt to “improve” the text ,making it support Catholic views about Jesus.  Here the sincerity is perhaps more in doubt!  In the case of Luke 22:43-44, it was an attempt to smooth the text, making it more like the other versions.  We can assume that the motives were more or less sincere in each case.  With the John 8 passage, we should probably be thankful, as it preserves a wonderful legitimate story in the life of Jesus but in the case of 1 John 5, although the interpolation does not create false doctrine, we probably should not appreciate the attempt to “improve” the text.  The same could be said for Luke 22:43-44.   Like I already said, Luke 22:19-20 and 1 Cor 14:34-35 almost surely were in the original, so there is no interpolation there to explain the motive of.

The main conclusion is this.  We have so much manuscript evidence that in all of these cases, we are able to reconstruct the original with near complete certainty. The fact that there were a small number of interpolations into the New Testament text is not significant simply because we know these were interpolations.  Therefore, the reason it happened is a good question, but in the end, it does not affect the reliability of the New Testament.  Add to this the fact that, even if we did not know these were interpolations (as was true two hundred years ago, for example), the fact is that no important Christian teaching is affected by these interpolations.  Skeptics who try to use the facts to create doubt about the reliability of the Greek New Testament fail to notice that all the evidence increases  rather than decreases a reasonable person’s faith in the reliability of the New Testament scriptures.

Your next question is a completely different one.  You are asking whether, indeed, the original, autographs of the New Testament are inspired by God.  I am convinced, and I have a LOT of evidence to support this, that the original New Testament writings are inspired by God.  Matthew was an eye witness to the events, and he did not invent the sayings or acts of Jesus.  John was an eye-witness as well, and the evidence is that his is a faithful rendition of what Jesus actually said and did.  These are NOT fabrications.  There is no evidence, for example, that any of the I AM statements in John are later additions.  You can be extremely sure of that.  The gospel writers had a genuine faith in Jesus and they had no motivation to lie about what Jesus said and did.  Why would a person who would die because a genuine faith in the actual Jesus make up false teachings or events?  Besides, we have four mostly independent versions of the events, which is a check on any possible willful errors.  Are our New Testaments perfect?  The evidence is that they are perfect or nearly so.  Faith fills in the gap between nearly perfect, which the evidence shows and perfect, which faith believes.

I hope this helps.

John Oakes