How do I know what or when to compromise to advance the cause of Christ?
The cause of God is not advanced by sin. "And why not say (as we are slanderously reported and as some claim that we say), "Let us do evil that good may come"? Their condemnation is just" (Romans 3:8). This is because God is without sin. "This is the message we have heard from Him and announce to you, that God is Light, and in Him there is no darkness at all. If we say that we have fellowship with Him and yet walk in the darkness, we lie and do not practice the truth" (I John 1:5-6).
However, within righteousness, there is still a good deal of flexibility. Thus, when it comes to personal preference, I must be willing to be flexible when dealing with other people. "For though I am free from all men, I have made myself a slave to all, so that I may win more. To the Jews I became as a Jew, so that I might win Jews; to those who are under the Law, as under the Law though not being myself under the Law, so that I might win those who are under the Law; to those who are without law, as without law, though not being without the law of God but under the law of Christ, so that I might win those who are without law. To the weak I became weak, that I might win the weak; I have become all things to all men, so that I may by all means save some. I do all things for the sake of the gospel, so that I may become a fellow partaker of it" (I Corinthians 9:19-23).