Matthew Poole's Commentary
The penman of this Epistle, viz. Paul, was so called (as some think) because he was little or low of stature. Others suppose he had this name first given him upon the converting of Sergius Paulus the deputy; of which see Act 12:1-25, and Hierom. Com. in Ep. ad Philem. But others are of opinion, that his name was not changed at all, and that he had two names, as all those Jews had who were freemen of Rome. The text in Act 13:9 doth favour this opinion; there you read of Saul, who was also called Paul. (So John was surnamed, or also called, Mark, Act 12:12, Act 12:25.) And because he was the apostle of the Gentiles, and his work lay mostly amongst them, he was called at last altogether by his surname, or Roman title.
As to the order of it: all are agreed, that it was not written as it is placed in our Bibles: that the Epistles to the Thessalonians, to the Corinthians, and other of his Epistles, were written before this; and the reason why it is placed before the other Epistles is, because of the dignity of the Romans, to whom it was directed; Rome being, at that time, the imperial city: or, because of the prolixity and largeness of it, this being the longest of all the Epistles: or, because of the excellency and fulness of it; so full and excellent is this Epistle, that some have called it "the marrow of divinity". Chrysostom had such an esteem of it, that he caused it to be read to him twice every week. Melancthon called it "the confession of the churches"; he is reported to have gone over it ten several times in his ordinary lectures. Mr. Perkins adviseth, in the reading of the Scriptures, to begin with the Gospel of John, and this Epistle to the Romans, as being the keys of the New Testament.
The subject matter of it seems to be much the same with the Epistle to the Galatians. The body of this Epistle (not to speak any thing of the preface, or conclusion) is partly doctrinal, and partly practical. In the doctrinal part, the apostle handles (and that purposely, and at large) that fundamental article of a sinner's justification in the sight of God: so that this Epistle (as one saith) is the proper seat of that doctrine; and from hence it is principally to be learned. Here we are taught the way and manner of our justification before God, that we are justified by faith, without the deeds of the law, by a righteousness which is imputed to us, and not by any righteousness inherent in us. This is proved in the first four chapters, by many irrefragable arguments, and vindicated from all objections. And then it is amplified in the seven following chapters. The amplification is first from the glorious effects and sweetest privileges of justification by faith, viz. peace with God, which no tribulation can hinder or interrupt, Rom 5:1-10. Then there is rejoicing with God, as reconciled through Jesus Christ, the Second Adam, who doth abundantly transcend the first Adam in many particulars, Rom 5:11-21. Then there is sanctification, in both the parts of it, as mortification and death to sin; and vivification, or newness of life, Rom 6:1-23 throughout. The next is freedom from the law, as the first husband, now dead, Rom 7:1-25. And in the Rom 8:1-39 you have divers other privileges closely couched, as noncondemnation, adoption, the indwelling of the Spirit, the co-operations of all things for good, the certainty of the love of God, together with the triumph we have over all our enemies upon that account. Further, this doctrine of justification is amplified from the remote cause of it; and that is, God's predestination or eternal counsel. This is brought in to obviate an objection against this doctrine, as not true, because the Jewish nation (God's ancient people) received it not. Thereupon the apostle shows, that justification belonged not to the whole nation of the Jews, but only to the elect amongst them; the rest being rejected of God till the fulness of the Gentiles was come in; and then the Jews should more generally believe and be converted. This you have at large in Rom 9:1-11:36. The practical part of this Epistle follows, in which you have many useful exhortations, from Rom 12:1-15:14. These are either more general, or more particular, showing Christians how they should behave themselves with respect to the church of Christ, and the fellowship thereof, every one attending upon the calling and ministry wherein God hath placed him, Rom 12:1-21; with respect to the civil society, and the government which God had set over them in the world, yielding all subjection thereunto, Rom 13:1-14; and with respect to their brethren and neighbours, exercising Christian charity towards all, avoiding censoriousness on the one hand and offences on the other, Rom 14:1-15:33. These duties he largely presseth, interweaving now and then many ethical and theological aphorisms, of which in their proper place.