1 Corinthians 6:12
12All things are lawful unto me, but all things are not expedient: all things are lawful for me, but I will not be brought under the power of any.
All Jews to whom Paul and others have preached the good news to are living under the Law. Jews who were living under the Law were granted an inheritance into the Kingdom provided they lived the right lifestyle (which would have been impossible). Now, Paul moves on from talking about the Kingdom and life under the Law to what their life is as Christians.
All things are lawful. What is permissible for a Christian? All things. This certainly cannot mean all things, can it? And yet, that is what Paul states. In other words, our salvation is not dependent on what we do. Remember the list in the previous verses? Doing any of those things will not keep a person out of heaven.
But, Paul continues, all things are not expedient, meaning some things will cause a considerable burden, so they are not profitable. They may not keep you out of heaven, but they will not bring any return on investment. The time spent doing certain things will not result in anything positive, they are a waste of time.
While all things are lawful for me, Paul makes it clear that he will not be brought under the power of any sin. He will not let unprofitable things master him.