The book of Romans is essential to the New Testament because it is powerfully written and presents the doctrines of the faith as clearly and directly as any other book in the Bible.
This is a book which has impacted readers of it through the centuries.
Luther, in his preface to the Roman letter, wrote: "This Epistle is the chief book of the New Testament, the purest gospel. It deserves not only to be known word for word by every Christian, but to be the subject of his meditation day by day, the daily bread of his soul.… The more time one spends in it, the more precious it becomes and the better it appears." He spoke of it as "a light and way into the whole Scriptures...." Calvin said of it "when any one understands this Epistle, he has a passage opened to him to the understanding of the whole Scriptures." Coleridge pronounced Romans "the most profound work ever written!" Meyer considered it "the greatest and richest of all the apostolic works." Godet referred to it as "the cathedral of the Christian faith." Gordon H. Clark recently wrote of Romans that it is "the most profound of all the epistles, and perhaps the most important book in the Bible.…" Hamilton, in his recent commentary on Romans, calls it "the greatest book in the Bible."
It is a letter written by Paul to a church he had never visited (though he spends several years with them later), yet he addresses many people by their names. So, while it is a personal letter in that regard, it is clearly written to an entire body of believers. He wrote the letter from Corinth and relates sage advice based on what he sees there (plenty of sin, it seems) as well what he thinks the Roman church should be studying and learning. Remember that, while modern Christians have the Bible in its entirety, the New Testament churches generally had little or nothing of the New Testament, so letters like this one really gave them the practical teachings of Jesus which were new and sometimes quite confusing to them.
Several of the most hard-to-understand doctrines of the faith are outlined in Romans better (arguably) than anywhere else in the Bible: justification by faith and the righteousness of God. Romans 1:16-17 says:
For I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes, to the Jew first and also to the Greek. For in it the righteousness of God is revealed from faith to faith; as it is written, “But the righteous man shall live by faith.”
Paul goes on, in the next ten chapters or so, to outline the foundational doctrines of Christianity; he finishes his letter (the book) with some practical advice about how to live out those principles on a daily basis. Chapter twelve begins with the word "so," indicating that all that came before is one thing (teaching) and what follows is another (practice). This is the foundation of all learning and growing in every area of life: first learn the principles and then apply them.
We are imperfect learners and appliers, but we have the book of Romans from which to continue learning and applying. It is an essential book of the New Testament.
Many of the supporting scripture verses for the Westminster Confession of Faith (an important document stating the basic principles of many protestant branches) are from the the New Testament, including the book of Romans.
For instance, in chapter four of the Westminster Confession the fourth principle is discussing the consequences of original sin and how all human beings are by extension subject to evil. One of the scriptures supporting this claim is Romans 3:10 which reads "As is it writen- 'There is no one righteous, not even one.'"
It would be difficult to support the Westminster Confession of Faith without the book of Romans and consequently many denominations would have to reevaluate their core principles.
In the Book of Romans Paul offers a particularly powerful testament of his conversion and vision of Jesus on the Road to Damascus. Because only a very select and small group of individuals supposedly saw the resurrected lord after his crucifixion, many individuals were skeptical of this major component of Christian theology. It was not until Paul wrote that he had seen the resurrected Christ on the Road to Damascus that Jesus having risen from the dead was widely accepted.
Chapter 8 of Romans marks one of these emotional climaxes. He writes that no form of trial or tribulation will separate any individual from the love of Christ, and that all can be conquerors.
Furthermore, the book of Romans is a valuable text because it is directed towards the Gentiles in a time when Christianity was spreading and there were many other converts, just as Paul was converted.