The story of Noah begins:
>The Nephilim were in the earth in those days, and also after
"and also after"
They weren't killed off by the flood. Joshua and other Israelites find them in Canaan. So, no, according to the scripture we all share, not everyone is descended from Noah.

>"But that's the Nephilim," some have argued. "A special case. But all normal humans are descended from Noah."
Look to the list of nations who are descended from Noah (Gen 10). We see from Gen 13 to Gen 15 tribes *not on that list*--even tribes that had been named prior to the flood and not listed as descended from Noah, here found well and alive after the flood.

We also see *two distinct sets of directions* to the Israelites of how to handle different tribes as they conquer the land of Canaan. The tribes of which they are instructed it is fine to take wives of *are on the list.* The tribes of which they are told to not take wives--to not even take livestock--*are not on the list* (or had mixed with the tribes not on the list, explicitly stated in scripture).

This same reasoning is why Abram had cared about his son having a wife of his own people, though living among Canaanites--going so far as to send a servant to his people to find one. And then Isaac after this, telling his sons they are not to take wives of the Canaanites.

Not doing this is the sin Esau committed, taking Hittite wives to anger his father and mother--so much so that Rebekah, his mother, laments what her life is worth if Jacob also were to do the same.

Why was Noah picked of all people?
>"Noah was a righteous man, and perfect in his generations: Noah walked with God."
"perfect in his generations"
Take a breath and think: What does "perfect in his generations" mean?--and right after an introduction that talks about the sons of God taking wives of the daughters of Adam.
This is not to say Noah was the only uncorrupted descendant of Adam. Of any that were "perfect in [their] generations," only Noah would be in the line of first born (following the order of first born son outliving their father's passing, Noah being the eighth from Adam), which is how he's "the eighth preacher of righteousness" as 2 Peter declares. (It is worth looking at how wildly our different versions "translate" that verse. They add whole terms, trying to piece together how Noah is "eighth." Some decide he was just "eighth on the boat." 🙄)

The flood did not kill everyone and was not meant to. It was meant for the descendants of Adam--to start over with Noah, perfect in his generations, eighth in the line of first born, the eighth preacher of righteousness--because God lamented that those of Adam were becoming corrupted *racially* (which results in irreversible corruption, as we see even today). That is the blasphemy of the Holy Spirit--the blasphemy of that which God had given only to Adam. One may blaspheme the Son of Man and be forgiven, but the blasphemy of the Holy Spirit will not be forgiven, as it cannot be undone. The children will not be of the promise. A bad tree cannot produce good fruit, a good tree cannot produce bad fruit, and no where is it said a bad tree becomes a good tree.

It matters because of *how* Christ's salvation works.

It is Christ who is now the preacher of righteousness, the mediator, after that same consideration of first born priests/mediators (or as Paul states it in Hebrews, "after the order of Melchizedek," a transliteration of the Hebrew for "righteous king"). It is Christ, being God, that is the true vine, by the Spirit, rather than Adam. Even Adam is--by the Spirit given to him by God (who is Christ)--a son. And so, being the descendants then of our Father, we are the branches--and only through Christ, who is called the way and the life *because of this.*

We are not found as cursed descendants of a forefather (Adam), who would be our mediator, a cursed and sinful one, who died and would be replaced by the firstborn surviving him. Nor are we found with any of that line of firstborn as our mediator, who would also be found corrupt and also pass to death. Instead the forefather of even Adam (as He gave to Adam His Spirit) has come as a son of man. God Himself is our Father, by this, the true vine. And the justification of the restoration/resurrection (however you want to translate it) will be by this consideration. He is the vine; those descended from Him are the branches.

"Every plant which my heavenly Father planted not, shall be rooted up."

@Xenophon you certainly made a great effort to say very little of significance. Most of what you say reads far too much into the text (especially about the Nephilim), and your errors multiply from there.

Allow me to make a case in point:

You say that Noah being "perfect in his generations" means he was racially pure. This is not what the original word means. The word translated as "generations" in the phrase you quote is not a matter of genetics but of time. In fact, the verse containing the quoted phrase uses two different words which are both translated "generation(s)": תּוֹלְדָה meaning genealogical descent, and דּוֹר meaning an era or the time span of a particular generation of people. The latter is the one used when describing Noah's righteousness. Noah was not good because he was "pure of race"; the passage says he was blameless among the other humans living at the same time he was living.