Maybe He is saying not to try to force the gospel on people who obviously don't want it and won't hear it. Not that we shouldn't still pray for God to open their hearts, of course we should! But as long as their hearts are hardened, arguing with them will only harden them more. It's like throwing seed on the road [Matthew 13:4][Matthew 13:19], they can't understand it and won't receive it, so Satan snatches it right up. If you'll notice, Jesus didn't spend a lot of time trying to convert the Pharisees and teachers of the law, who thought they were already righteous in their own right (although He was open to them when they truly wanted to repent). He went to those who were hungry for Salvation.