What is the sin that does not lead to death?

The passage which mentions the sin that does not lead to death — as well as the sin that leads to death — is 1 John 5:16-17.  When one examines the immediate context of 1 John 5:13-18, and also examines 1 John 1:7-9, it is clear John was talking about sins Christians commit which are forgiven by God because the Christians walk in the light, confess their sins before God penitently and thus do not keep on sinning willfully and rebelliously (cf. Heb. 10:26-31), and thus can know for certain that eternal life is theirs.  Sins of which faithful Christians confess and repent are sins that do not lead to death.  Sins which unfaithful Christians willfully and rebelliously commit without repentance stay unforgiven, and thus lead to death (cf. Rom. 6:23).