Answer: Jesus said that the only sin that God would not forgive was blasphemy against the Holy Spirit, which was attributing to Satan the miracles which Jesus did in the power of the Holy Spirit (Matthew 12:22-32).
This sin would not likely be committed today because Jesus is not doing His miracles today as He did when He was here on earth. The unpardonable sin today is to not trust Christ as Saviour. God cannot save one who will not believe on Christ. Jesus said to the Jews: “I said therefore unto you, that ye shall die in your sins: for if ye believe not that I am He, ye shall die in your sins” (John 8:24).
"Study to shew thyself approved unto God." 2 Timothy 2:15