One of God’s characteristics is His righteousness. It is a characteristic that is foreign to man. This righteousness is reflected within His laws.
Romans 7:12 Wherefore the law is holy, and the commandment holy, and just, and good.
We see that the righteousness of God is reflected within the law that He has given to man. The problem with man is that he is not righteous. Man is not holy or just or good. Why would man being righteous be important? It is because God loves that which is righteous, and conversely hates that which is not.
Psalms 33:5 He loveth righteousness and judgment: the earth is full of the goodness of the LORD.
God loves that which is holy and just and good. Man cannot claim to have this trait of righteousness. Man cannot claim to be that which God would love in an of himself. The law of God was given to reveal what was required to be righteous. Righteousness must come to man and that is our problem before God. Job would ask:
Job 9:2 I know it is so of a truth: but how should man be just with God?
That is the fundamental problem man faces. How can he be right with God? How can God look upon man as holy and just and good? How can man who is said to only offer filthy rags to God in Isaiah 64:6 approach unto God? How can man who Paul says of which none are righteous in Romans 3:10 be right with God?
Romans 8:4 That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit.
The answer is the fulfillment of righteousness. Righteousness is that which remains when the law of God is fulfilled. What Paul is telling us in Romans 1:16-17 is that righteousness has come, it has been fulfilled. Righteousness is revealed to us in the very word of God. Man could not come to God, but God came to man. Jesus Christ, the Godman, has come and fulfilled righteousness. If He fulfilled righteousness in representation of you, then you are seen by God as just as righteous as He. That is how man is made right with God. That is how he can be seen as holy and just and good. This was done by no righteousness of our own, but through the obedience of Jesus Christ our Lord. Paul was telling us that all this has occurred. There is nothing more to be done. If it were not done then it could not be revealed. You cannot reveal that which does not exist. The righteousness we needed has already come by Christ’s obedience unto death. We live by that righteousness. We have life in Him. You cannot rightly understand the gospel without understanding this fact, the righteousness we need has already manifest therefore it can now, at the time of this writing, be revealed. Prior to its existence it could not be revealed. God was declaring how He could declare sinners righteous. This is the good news. If God required any addition by the sinner that could not be seen as good news. If God who requires perfection required anything from the sinner how much hope could we have of satisfying God?