What is the sin unto death it talks about in 1 John 5:16?
There are various texts that suggest hell exists. For example, we have the parable of Jesus about the rich man and Lazarus in Luke 16:19-31. That parable makes evident that this life is the place of decision. How we believe and behave here will determine our eternal destiny. The parable suggests that once you go to hell, you cannot come back. In other texts, Jesus refers to hell as Gehenna, an Aramaic term based on the name Hinnom, the valley just south of Jerusalem where garbage was regularly burned (see Matthew 18:8-9). The image of hell as similar to Gehenna–a place where the fire does not go out and the worm never dies–suggests a very unpleasant place (see also Matthew 13:42, 50).

It appears to be the sin of apostasy, of the rejection of God’s grace and love. It can also be called the blasphemy of the Holy Spirit (Mark 3), in which people call evil good, and good evil.
—Ben Witherington III