Rom. 1:14 I am debtor both to the Greeks, and to the Barbarians; both to the wise, and the unwise.
Paul is opening up his letter to the Romans here, and in this verse he is prefacing the statement that he is ready to preach the gospel wherever he is called upon to do so. The reason he feels so ready to preach is because he sees himself as a debtor. His debt is actually not to the Romans or Greeks, it is not the wise or unwise that he owes anything to. His debt is to God, he is indebted to Jesus Christ for the salvation of his soul. However there is no way he can repay that debt to God, it is not possible for him to make it up to God, there is no way he can give God anything God does not already have, especially something as important as the soul. Therefore if Paul wants to begin to repay this debt to God, he must be a servant to God for the rest of his life, and bring this saving gospel to all men.
We too, have been saved by the power of Christ, and have no way to repay this debt. Paul says in Rom. 12:1 "I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service". That "therefore" is referring to the previous chapter where he explains that we have been saved by the mercies of Christ, and therefore are to live accordingly. Please do not misunderstand me here, we work for Christ because of our salvation, not for our salvation. Paul also says "For to me to live is Christ, and to die is gain." He is a debtor to Christ and because of this debt he preaches the gospel to all men.
Secondly, I want you to see his obligation to bring this gospel to all men. Paul says in Rom. 10:14 "How shall they believe on him whom they have not heard, and how shall they here without a preacher?" And again in I Cor. 1:21 "It pleased God by the foolishness of preaching to save them that believe." Doing this will not repay God for what He has done, it will not benefit Him, God is holy and perfect and can not be benefited by the actions of mere men. However, Paul had the privilege of hearing this gospel preached and now knows he is obligated to give every man possible the same opportunity. That is not to say that everyone that hears will be saved, but that those whose spiritual ears God has opened, will believe unto salvation.
Now with these two principles in mind, let us come to the applicable conclusion. If you are saved, it is because you heard or read the gospel of Christ, and if someone went out of their way to give you that gospel, shouldn't you do the same? God has saved you regardless of who you were, or what you could give back to Him. You are now a debtor the same as Paul was, you owe it to everyone you come in contact with to proclaim the gospel and give them the chance to believe on Him of whom they have just heard. It is your reasonable service to present your bodies to God as a living sacrifice and preach His gospel, no matter how foolish it sounds to the lost. Believe me, to them it will sound foolish, Paul says in I Cor. 1:18 "For the preaching of the cross is to them that perish foolishness; but unto us which are saved it is the power of God." Start paying on that debt, start preaching the gospel to every creature