What is the one sin that God can never forgive? 

Blasphemy against the Holy Spirit.

Blasphemy against the Holy Spirit is often understood as a sin of persistent unbelief which is willful rejection of God’s grace and the miraculous work of the Holy Spirit, which hinders the possibility of forgiveness.

Mathew 12:32

“Whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.”