Notice that Paul very carefully guards against an "anything goes" mentality by what he says parenthetically in verse 21. He is under the law of Christ! He is not free to do anything he wants (or as it is sometimes put, "whatever it takes") for the sake of seeing people saved. He is Christ's man and must live according to the law of His Lord. He will not sin in the attempt to get people saved. He will not act in a way that is unworthy of Gospel for that purpose either.
I am convinced that it is at just this point that many Christians--including Christian leaders--go astray in their well-intended but misguided zeal to do outlandish, even unscrupulous things to see people converted or to grow a church. They have lost sight of God's law and, consequently, of the seriousness of sin. They see the noble goal of getting people converted, and assume that "whatever it takes" is OK in pursuing it. When challenged about the biblical appropriateness of their methods, the argument tends to be, "but look at the results."
Paul's testimony in Romans 7:13 needs to be reconsidered. He describes one purpose of the law by saying that it functions so that "sin through the commandment might become exceedingly sinful." The law is a transcript of God's character. His holiness is displayed in His moral commandments. If we would remember His holiness and regularly remind ourselves of the sinfulness of sin we would shrink from any activity that would violate His standard of righteousness, no matter how noble the goal.