The license to sin argument is old. 2,000 years old. It's an evil heart of unbelief, an antichrist spirit, that attacks the gospel.
Satan will present some offensive sin, and suggest that the grace of God won't forgive that one. It's usually something that the accused would indeed find offensive and have no desire to do. But acquiescing implies that the finished work of Christ wasn't finished, that the gospel is untrue, and God is a liar. Jesus died for all sin, even the ones we may find most offensive.
Even if it seems unfair to the mind of man, it is lawful. By Jesus act of obedience to His father, by His finished work on the cross, He paid the price for all sin. Not some, all. And He offers complete forgiveness and salvation for everyone who will believe on Him. Without exception.
For those that reject this free gift Jesus has offered, they can explain to Jesus how unfair it is, just as they're cast into the Lake of Fire to pay their own debt.
Jesus paid the price, and it's His right to offer forgiveness and salvation freely.
And for those of us who have believed, who have the grace of God upon us, many of us have come to understand the need for the grace of God, every day, in every moment. I don't just need it sometimes. I need it all the time. I need it all the time, not because I want to live badly, but rather so I can walk forward unafraid, knowing God is always with me, and always for me, even when I fall short. Because we all do. And having received grace, I also desire to give it to others.
How about the ambassadors and diplomats in this world? Would they also complain that the diplomatic immunity they've been given, because of who they are, is a license to commit crime? Will they lie awake at night, imagining all the crimes they're getting away with, and how unfair it is? It is true, every once in awhile you hear of someone with diplomatic immunity committing a big crime and getting away with it.
And would they rather say they have a license for crime, instead of diplomatic immunity? That would take the focus off diplomacy, and put it on crime, wouldn't it?
When an agency issues a license, they are granting approval of the thing the license is for. By giving us complete forgiveness, God is not approving of any wrong thing, but rather forgiving it, even though it's wrong. So calling the grace of God a license to sin would be accusing God of approving of evil. The grace of God is His loving mercy towards us, giving us forgiveness, not because we're deserving, but because He loves us. Not approving of any wrong thing, but forgiving us and loving us, despite every thing. Not according to the wisdom and ways of man, but by His wisdom, and His goodness.
So the argument, that the gospel, and the grace of God is a license to sin, is not logical. For those that reject the gospel and the grace of God, and rather preach a false gospel of self righteousness, perhaps we should ask, if satan has given them a license to lie and deceive?