Suppose a state were to allow a man to marry his father's wife. They would be legally "married".
But such a union is prohibited by God (
Deut 22:30). This prohibition is recognized as valid in the New Testament (
1 Cor 5:1, where it is specifically called fornication). Since such unions are prohibited by Divine edict, no amount of state paperwork can make them legitimate.
Now, suppose such a man becomes a Christian. He divorces the woman, and is now legally a divorced man. How is the church to relate to him?
According to the state, he was married, and is now divorced. According to God, he was a fornicator, and has now repented. Thus, the question "Can he marry a woman?" should be answered by the church on the basis of God's Word, not the state's. He never was actually married, anymore than two men could be, or a man and his dog, or a man and himself, could be married.