I'm asking about Rom 3 and 4. So far I take it you see Rom 3:31 'establishing the law' as a result of our walk.
Abraham in Romans 4 faith establishes the law.
Hebrews 11:8-9
8 By faith Abraham, when he was called to go out into a place which he should after receive for an inheritance, obeyed; and he went out , not knowing whither he went.
9 By faith he sojourned in the land of promise, as in a strange country, dwelling in tabernacles with Isaac and Jacob, the heirs with him of the same promise:
Hebrews 11:24-30
24 By faith Moses, when he was come to years, refused to be called the son of Pharaoh's daughter;
25 Choosing rather to suffer affliction with the people of God, than to enjoy the pleasures of sin for a season;
26 Esteeming the reproach of Christ greater riches than the treasures in Egypt: for he had respect unto the recompence of the reward.
27 By faith he forsook Egypt, not fearing the wrath of the king: for he endured , as seeing him who is invisible.
28 Through faith he kept the passover, and the sprinkling of blood, lest he that destroyed the firstborn should touch them.
29 By faith they passed through the Red sea as by dry land: which the Egyptians assaying to do were drowned .
30 By faith the walls of Jericho fell down , after they were compassed about seven days.
By faith they established the law, and by faith we establish the law. It's really simple.