Commentary of Charles Spurgeon, Adam Clarke & Matthew Henry
Now I would not have you ignorant, brethren, that oftentimes I purposed
to come unto you, (but was let hitherto,) that I might have some fruit
among you also, even as among other Gentiles.
I am debtor both to the Greeks, and to the Barbarians;
both to the wise, and to the unwise.
So, as much as in me is, I am ready to preach the gospel to you that are at Rome also.
For I am not ashamed of the gospel of Christ: for it is the power of God
unto salvation to every one that believeth; to the Jew first, and also
to the Greek. For therein is the righteousness of God revealed from faith to faith: as it is written,
The just shall live by faith.
Vs.13-17
That is the sum and substance of the gospel: “The just shall live by faith.” 
But was let hitherto -...likely that the original word, εκωλυθην, I was forbidden, refers to a Divine prohibition.
I am a debtor both to the Greeks, and to the barbarians - It has been remarked before that all the nations of the earth, themselves excepted, were termed barbarians by the Greeks.
I am ready—prothymon.
It denotes a great readiness of mind.
I am not ashamed of the Gospel of Christ - Paul was not ashamed to own it.
To the Jew first - Not only the Jews
have the first offer of this Gospel, but they have the greatest need of
it; being so deeply fallen, and having sinned against such glorious
privileges, they are much more culpable than the Gentiles, who never had
the light of a Divine revelation.
And also to the Greek - Though the
salvation of God has hitherto been apparently confined to the Jewish
people, yet it shall be so no longer, for the Gospel of Christ is sent
to the Gentiles as well as the Jews; God having put no difference
between them; and Jesus Christ having tasted death for Every man.
The just by faith, shall live; that is, he alone that is justified by faith shall be saved.
