Unfortunately many people struggle with understanding the difference between the Old and New Testament. For many people the only difference is one blank page dividing the two covenants. But these difference are far greater then the span between Malachi and Matthew.
This misunderstanding can be attributed to not comprehending a passage of scripture as this. At first glace it appears that Paul is saying that to be declared righteous before God, one must obey the law. This would lend towards the marriage of the two covenants as one. However, unless one understands the purpose of the Law and why it was given, this line between becomes fuzzy and understanding the differences will not come.
The sole purpose of the Law was to point us towards Christ. The law showed us the righteous requirements of God. The law showed us what sin was. The law showed us that we are completely incapable of obtaining right standing with God by our deeds alone. We are hopeless to fulfilling the law and need to fall at the mercy of God and His saving grace, which was the intent from the beginning.
Therefore, the purpose of the law shows us our need for a Savior and subsequently points us to Christ as our the Messiah. By accepting Christ, we are fulfilling the law. By obeying the purpose of the law, which points us to Jesus, we will be declared righteous by Him before God.
Looking at the entire book of Romans in relation to this passage of Scripture, Paul says the same thing. "Now we know that whatever the law says, it says to those who are under the law, so that every mouth may be silenced and the whole world held accountable to God. Therefore no one will be declared righteous in his sight by observing the law; rather, through the law we become conscious of sin. " ( Romans 3:19-20 ) In only a few short verses after today's passage, Paul confirms that the purpose of the law was to bring us to awareness of the need of a savior.
He goes onto say, "But now a righteousness from God, apart from law, has been made known, to which the Law and the Prophets testify. This righteousness from God comes through faith in Jesus Christ to all who believe. There is no difference, for all have sinned and fall short of the glory of God, and are justified freely by his grace through the redemption that came by Christ Jesus." ( Romans 3:21-24 ) First he says in our passage today that those who obey the law will be declared righteous. Then in Romans 3 he describes how to obey the law. He says, that the righteousness that the law and prophets testified about comes through faith in Christ Jesus. So we can conclude that the entire Old Testament law brings us to faith in Jesus.
Understanding this, is key to understanding and unlocking the truth of God's word. 'As the Scripture says, "Anyone who trusts in him will never be put to shame." ( Romans 10:11 ) Today, may you dwell in the righteousness purchased for you. May you understand that you have peace with God through our Lord Jesus Christ. May this revelation unlock the fullness of Christ and propel you towards His plan for your life!