Charles D. Myers, Jr., writes on the place of Romans in the genuine correspondence of Paul (The Anchor Bible Dictionary, v. 5, p. 817):
A relative chronology of the Pauline Epistles can be constructed by means of references in Paul’s genuine epistles to the Jerusalem collection. This collection was inaugurated at the apostolic council described in Galatians 2, when Paul agrees to “remember the poor” (Gal 2:10). The collection was introduced to the Corinthians in 1 Cor 16:1-4, where Paul provides directions for collecting money. Then in 2 Corinthians 8-9 (esp. 8:6, 10; 9:1) Paul exhorts the Corinthians to complete what they have begun. When Paul writes Romans, he is ready to travel to Jerusalem with what has been collected among the gentile believers in Macadonia and Achaia (Rom 15:25-26). Prior to the time of writing Romans, therefore, Paul had already written Galatians, and 1 and 2 Corinthians, in addition to 1 Thessalonians (believed to be Paul’s earliest extant epistle), and perhaps Philippians as well.
Myers writes on the relationship between the ideas in Romans and in his earlier epistles (op. cit., v. 5, p. 817):
Moreover, Romans is heavily indebted to those epistles that have gone before. As G. Bornkamm (1963: 2-14) has rightly pointed out, a number of topics that are present in Paul’s earlier epistles surface in Paul’s Epistle to the Romans. Among those topics are justification by faith and not by the works of the law (Galatians 3-5; Philippians 3; Romans 1-4); the fatherhood of Abraham (Galatians 3; Romans 4); Adam as the head of the old order of humanity and Christ the head of a new order (1 Cor 15:21-22, 45-49; Rom 5:12-19); the church as Christ’s body composed of diverse elements (1 Corinthians 12; Rom 12:4-8); the need to exercise personal freedoms with consideration for the consciences of others (1 Corinthians 8-10; Romans 14-15) – to name only a few. But in Romans, Paul does not merely reiterate these ideas; rather he reformulates and refines them. Romans, therefore, evidences a greater theological maturity than the other Pauline epistles.
Myers comments on the importance of Romans (op. cit., v. 5, p. 817):
The Epistles to the Romans has also contributed significantly to the history of Christian doctrine. Almost every influential Christian thinker has dealt with Romans. Origen, Thomas Aquinas, and Philip Melanchthon, to mention only a few, wrote noteworthy commentaries on Romans. And numerous theological notions have been derived solely or in part from Romans. Augustine acquired his idea of original sin from Romans 5, Luther gained his understanding of justification by faith alone from Romans 3-4, John Calvin obtained his doctrine of double predestination from Romans 9-11, John Wesley got his distinctive teaching on sanctification from Romans 6 and 8, and Karl Barth learned of the importance of the righteousness of God from Romans 1 and 2. In short, this epistle has exerted a powerful influence on all branches of the Christian Church, and its impact on the lives and thought of prominent Christian thinkers through the years has been second, perhaps, only to the canonical gospels.