Our tongue is to bless God, not insult Him
not misuse the name of the Lord your God, for the Lord will not hold anyone guiltless who misuses His name” (Ex 20:7).
Sinning defiantly by deliberate failing to obey or be truthful is blaspheming the Lord (Num 15:30,31; Act 5:1-10). Saul (before his conversion and name change to Paul) was a blasphemer and tried to force the early Christians to blaspheme, and renounce their faith in the God (Act 26:11; 1 Tim 1:13). Others were rebuked as “God’s name is blasphemed among the Gentiles because of you” (Isa 52:5; Ezek 36:22; Rom 2:24).
However, Jesus said all the sins and blasphemy of men will be forgiven, except speaking evil of the Holy Spirit by attributing the works of Jesus to Satan which is rooted in unbelief (Mt 12:22-32; Mk 3:22-30). The only sin God can't forgive is unbelief – the refusing to turn to Him for forgiveness.