What is “sin that leads to death” in 1 John 5:16?

There are a whole lot of sins enumerated where the perpetrator was to be put to death. They are all summarized in this verse:

   Deuteronomy 30:19
   I call heaven and earth to record this day against you, that I have set before you life and death, blessing and cursing: therefore choose life, that both thou and thy seed may live:

The opposite choice would be to sin (any sin) and thereby reject God. That is the sin which leads to death if it is continued in.

   Hebrews 4:7
   Again, he limiteth a certain day, saying in David, To day, after so long a time; as it is said, To day if ye will hear his voice, harden not your hearts.

Sins which do not lead to death are those sins of the flesh done by instinct. They are not done by intention. Believers wash in the Word and are 'unclean until the evening'.