Paul’s religious critics did not understand why religious observance was not enough to win favour with God.  They thought Paul was saying that free salvation through faith in Christ meant that God is not offended by sin.  They claimed that the gospel promoted sinning.  Surely, they argued, if Christ is pleased to forgive sin then He will be pleased when people sin more!  If that was true, then He is a promoter of sin, and a law-breaker Himself!
 
Why should Jesus Christ, who suffered so intensely because of our sin, want to encourage it anymore?  In Romans 6:1-2, Paul wrote about that false teaching: “What shall we say, then? Shall we go on sinning, so that grace may increase? By no means! We are those who have died to sin; how can we live in it any longer?”   What is an appropriate response to the suffering of Christ: to deliberately sin more?  No!  That would show that we had no gratitude or love towards our Saviour.  The only right response is to repent, repudiating all that is wrong, and live in a way which will please Him.
 
The idea that ‘God does not mind sin’ can creep into Christian minds, even if it is not explicitly taught.  It is one of Satan’s tools to disable God’s people, making them ineffective as agents of light in a world of darkness.  Sometimes the Evil One even stirs leaders to teach that God does not mind what we do, because He loves us and wants us to be happy. They are wrong.  

God is far more zealous for our moral purity than we often imagine.  A real friendship is two-way: Christ suffered for us and we live to please Him.  When that link is broken, there is little functional relationship.  So let us be sure that we live right and teach the truth while humbly trusting in the grace of Christ.