Matthew 7:18 A good tree cannot bear bad fruit, nor can a bad tree bear good fruit.
Have you ever got caught up in activities that are not lawful or not right? I know that I have been caught up in many things in my life and have turned a blind eye in many cases too; but each time my conscience told me that it was wrong!
When Jesus used these words it was to remind us that we do give in to the things around us through giving in to temptations, but those actions do not come from inside of us.
What we have in our hearts will come out and show itself and Jesus wants us to have His ways and His heart inside of our hearts.