Isn't the Christian supposed to defend biblical marriage in the courts and legislatures? Doesn't the Christian have a mission to oppose society's departure from God's moral law?
Before we address the heart of your question, we need to clarify what the Law of God’s is. God’s law is not limited to a set of “moral” laws within the overall Mosaic Law, as some maintain. In scripture, there is no distinction between a “moral” law and any other part of the law. As James said, if we break one law, we are guilty of the whole law. Either a person keeps the entire law, or else there is no advantage in keeping part of it.
In reality, there is only one Law, which consists of all 613 commandments, statutes and ordinances given to Israel by Moses. Jesus Christ kept that Law perfectly so that He could meet its terms on our behalf. When we come to faith in Jesus Christ, we fulfill the requirements of God’s Law through our reliance on Messiah's sinless life and His atoning death in our place.
Therefore, the only way we bring the world into conformity with God’s Law is through the same means we were conformed ourselves: by faith in Jesus Christ.
As Jesus Himself instructed us, the mission of the church is simple:
As the Great Commission directs, the Church is called to make disciples of Christ, which implies recruiting them to Christ by sharing the Gospel. Once a person accepts Christ as Lord, then we continue serving them by teaching them to obey the commands of Christ. This is the mission of the Church, and as we fulfill this mission, God’s elect will hear, believe and be conformed to the image of God.
Notice, Jesus gave no commands to teach the world to obey Christ’s commands but only to teach those who were already His disciples by faith, for obedience apart from becoming Christ's disciple through faith is impossible. Only by believing in Christ does someone have the potential to live in agreement with the Law of God. In fact, the scriptures teach it is impossible for fallen, sinful humanity to conform to the word of God:
The unbeliever is living in the flesh and not by the Spirit, therefore they lack the ability to conform to God’s Law, while those who are saved are led by the Spirit of God into agreement with the Law. Only by faith in Jesus Christ can anyone meet the terms of God’s Law, and by that same faith we are equipped to live according to its precepts as we are sanctified by the Spirit.
Moreover, the Church is called to testify to our faith by our lifestyle choices. As we live out our faith in obedience to Christ's commands, we show Christ in us and represent His holiness before the world. Just as the tabernacle shined the light of God’s glory in Israel, so does the Church shine the Light of Christ in the world today.
Therefore, we should anticipate that our lifestyle and biblical viewpoint will often come into conflict with world’s views (as in the case of marriage), and in those circumstances the believer must stand firm in the truth and suffer whatever consequences come as a result.
If God did not spare the apostles and early church fathers – and did not even spare His Son – then we have no reason to expect Him to spare us from persecution either. In fact, the scripture says plainly we will be persecuted and we should rejoice in it:
As the world moves away from the biblical view of marriage, the Church must continue living and teaching the Bible’s perspective on marriage while understanding that we will be at odds with the world. We will likely suffer persecution for these beliefs. We accept these outcomes as the natural consequences of following Christ, because persecution is the unavoidable result of representing Christ before His enemies.
No one comes to faith because of another person’s holiness. A heart is converted because it is first convicted of sin, repents and receives the love of Christ through faith, but this process can be furthered in God’s economy by the testimonies of godly men and women who suffer for their faith. Therefore, we preach the truth, we live the truth, and we suffer the hatred of a world convicted by what they hear and see in us in the hope and expectation that the Lord may save some through our obedience.
Clearly, the Church cannot accomplish this mission through political speech or legislation action. Instead, we accomplish it by the word of God. We preach the Gospel that lives may be transformed, we teach the word so that disciples may become obedient to the words of Christ, and we suffer gladly as the Lord appoints.
As we do these things in humility and love, we will impact the world, recruiting some for the kingdom. In summary, the church must remain focused on our Christ-given mission of winning souls for the kingdom without becoming distracted by vain and fruitless efforts to legislate the kingdom’s standards upon the fallen world.
When a ship is sinking, we don’t strike up the band to cheer on the doomed passengers. Instead, we get as many people as we can off the ship and into the lifeboats. That’s the loving response to impending disaster.