The Bible tells us that beings of giant physical stature lived both before and after the Flood (Genesis 6:4; Deuteronomy 2:10–12, 20–23). After the Flood, giants appear in the Bible as descendants of Ham’s son Canaan; they were living in Canaan when Moses sent agents to spy out the land (Numbers 13:1–2, 32–33).
Og, King of Bashan, was the last recorded giant east of the Jordan River (Joshua 12:4; 13:12). After Joshua’s conquest of Canaan (Joshua 11:21–22), a few giants continued living in the Philistine cities of Gaza, Gath and Ashdod. These giants terrorized the Israelites from the time they entered into Canaan until late in the reign of King David. One of those giants was Goliath, whom David killed. King David and his "mighty men" also encountered other giants (2 Samuel 21:16–22;1 Chronicles 20:4–8).
Some point to a passage in Genesis as evidence that these giants were the offspring of angels mating with human women: "Now it came to pass, when men began to multiply on the face of the earth, and daughters were born to them, that the sons of God saw the daughters of men, that they were beautiful; and they took wives for themselves of all whom they chose" (Genesis 6:1–2).
It is important to realize that although God sometimes does call angels His sons (Job 38:7), the Genesis 6 description of "sons of God" cannot be referring to angels, because angels were created as spirit beings. Each angel was brought into individual existence, complete, without the need or the ability to reproduce. As spirit beings, angels cannot mate with human beings. We know this from Jesus Christ’s own words, describing what happens when mortal human beings are born as spirit beings at the resurrection: "And Jesus answered and said to them, ‘The sons of this age marry and are given in marriage. But those who are counted worthy to attain that age, and the resurrection from the dead, neither marry nor are given in marriage; nor can they die anymore, for they are equal to the angels and are sons of God, being sons of the resurrection’" (Luke 20:34–36; cf. Matthew 22:30; Mark 12:25). Only God the Son, who has all creative power, has ever crossed the gap between spirit and flesh, which He did when being born as Jesus Christ (Philippians 2:7).
To claim that angels had the ability to create flesh (the giants) from spirit is not only illogical—it is blasphemy.
Who, then, were the sons of God mentioned in Genesis 6? They were powerful and famous men whose offspring, before the Flood, became famous in their time. Scripture does not tell us specifically, but many Bible scholars believe they could be the godly seed of Seth. Another important detail we should notice is that Scripture does not say that the giants originated from the unions described in Genesis 6; rather, the giants already existed at that time. "There were giants on the earth in those days, and also afterward, when the sons of God came in to the daughters of men and they bore children to them" (Genesis 6:4).
These giants, known as the Nephilim, did not survive the Flood. The existence of other giants known as Rephaim was noted after the flood, but even these giants disappear from the biblical accounts from approximately 1000bc onward, or after the time of King David. In any case, Scripture gives no indication that these giants were anything other than large human beings who will share in the same ultimate destiny as all other human beings.