If we assume that the Nephiliim were real, we can be fairly certain that they would no longer exist today. We can be certain about that because of the flood. Looking at the build up to Noah’s flood, we are first introduced to the nephilim. They are part of the reason why God becomes dissatisfied with creation, and is seeking to erase it. So part of the reason for the flood was to also get rid of such an “abomination.” We can thus be certain that neither the line of Noah, or his wife, or sons’ wives, were connected to the nephilim.

However, the idea of fallen angels, or angels cast from heaven, doesn’t stop at the nephilim. The Enochic pseudepigrapha contains other such stories. Lester L Grabbe has a work that deals with this, called A History of Jews and Judaism in the Second Temple Period. He argues that the idea of women and fallen angels having intercourse is a very old legend in Judaism. The debate is whether this legend informed the story in Genesis, or if the legend comes from the story in Genesis.

This idea actually continues through to Early Christianity and the rise of Rabbinic Judaism. It isn’t more until the third century that the idea that giants were produced from women and angels having intercourse really were really rejected. It was at this time that the Enochian writings were being pushed out as more heretical. However, it is worth noting that even the early church fathers took the idea of sons of God as being identified as fallen angels. Augustine of Hippo also shows the spread of this idea; however, he attests to it because of his polemic against such an idea.

In the third century, with the shift away from the Enochian writings, and this older idea of fallen angels, we see Christianity more adopting the view that the fallen angels were actually the ones led by Satan, and these fallen angels then become seen as demons.

So while the Biblical story of the Nephilim end up with them dying in the flood (or that is the implication), the idea of fallen angels and women having intercourse didn’t die out until much later, and then it was partially transformed into the Christian idea concerning Satan and his fall.

As for Goliath, I do not believe there are traditions stating that he was the product of a fallen angel. Later Jewish tradition, for instance, found in the Babylonian Talmud, he is said to be the son of Orpah. This is interesting as Orpah is the sister-in-law of Ruth (Ruth being the great grandmother of David).

The Jerusalem Talmud states that Goliath had a hundred or so father. The idea is that his mother was impregnated by a whole host of men, who were all his fathers. I think we can see the polemic here. An ungodly birth for sure, but not one from a fallen angel.

Now, Goliath was said to be a giant, but it wasn’t unnaturally tall. The Dead Sea Scrolls, and Josephus (I believe this is also true for the Septuagint) puts his height to around 6’9”. Other texts, such as the Masoretic text, put him more at 9’9”. However, scholars generally agree that in this case, the 6’9” is the older and better reading.

Follow up question: What about Numbers 13, there are very clearly Giants there (v.22, 28, 32-33) Anakim are a race of Nephalim after the flood. (Deut. 2:10, 20-21)

There is some debate here, but first, giants and the Nephilim are not interchangeable. Goliath, for example, was said to be a giant, but was not associated with the Nephilim in anyway. So we have to be careful there not to conflate the to ideas. At the same time, sometimes the term Nephilim referred to giants, even though they weren’t fathered by a fallen angel. Nephilim, at a certain time, seems to have been the term used in regards to giants, even though they weren’t considered to be products of fallen angels.

So on to the Anakim. In Numbers, the Anakim are said to be from the Nephilim. They aren’t said to be a race of Nephilm though, just that they are from them. Actually, what is says is that Anak, whose sons are called the Anakim, was from the Nephilim. So it is only talking about Anak himself. Note: There is a Hebrew tradition that states this differently. Where it talks about the Anakim being Nephilim. However, this isn’t found in the Greek, and many scholars believe that it may have been added later, as almost an in text note.

But then we start running into problems. So in Deuteronomy, all we are told about the Anakim is that they were tall. We are also told that they are counted as Rephaim, or Emim. We aren’t told much about the Rephaim, except that they were tall (they are never said to be Nephilim though) and that they existed far in the past.

The only possible connection with the Anakim and the Nephilim is Number 13:33. Every other mention of the Anakim, while they mention their height, never connect the two. Instead, they are connected to the Rephaim, who, while seen as legendary in their stature but far in the past, are not said to be Nephilim. And the key portion of Numbers 13:33 is a later addition, an added gloss, basically an in text note. Without the gloss, we get that Anak, who the tribe of Anakim descend from, is said to be from the Nephilm. How this actually works isn’t said, because we are introduced to some ancestors of Anak, specifically his father Arba, who isn’t said to be Nephilim, and aren’t related to them, as far as we know.

So the information is contradictory at best. The one clear connection is fraught with issues, mainly that it is a later addition, a gloss. What this gloss also implies though is that later on, the term Nephilim was being applied to those individuals who were seen as giants. The Anchor Bible Dictionary, when speaking of Nephilim, talk about how in later translations, the term almost became a generic term to signify various heroic traits.

It also mentions that there have been arguments that the original reason for the flood was the Nephilim as a race. The flood was meant to simply wipe them out, but later additions to the text expanded the reason. Either way, at the core of the flood, there was the intention to wipe out the Nephilim.

Second follow up: What about Numbers 13, there are very clearly Giants there (v.22, 28, 32-33) Anakim are a race of Nephalim after the flood. (Deut. 2:10, 20-21)