8 First, I thank my God through Jesus Christ for all of you, because your faith is proclaimed in all the world. 9 For God is my witness, whom I serve with my spirit in the gospel of his Son, that without ceasing I mention you 10 always in my prayers, asking that somehow by God's will I may now at last succeed in coming to you. 11 For I long to see you, that I may impart to you some spiritual gift to strengthen you— 12 that is, that we may be mutually encouraged by each other's faith, both yours and mine. 13 I want you to know, brothers,  that I have often intended to come to you (but thus far have been prevented), in order that I may reap some harvest among you as well as among the rest of the Gentiles. 14 I am under obligation both to Greeks and to barbarians,  both to the wise and to the foolish. 15 So I am eager to preach the gospel to you also who are in Rome. (Romans 1:8-15 (ESV)
I am under obligation both to Greeks and to barbarians, both to the wise and to the foolish.
Here we perceive the psychology of Paul. Preaching the gospel isn’t something he is indifferent too. He feels indebted to the gospel, it is an obligation he has to preach it.
Paul was a persecutor of the faith. His sole purpose was to get people to deny faith in Christ. Now he himself believes and his guilt drives him to preach to others.
I don’t know. We all know that guy who just quit smoking. Annoying as all get out isn’t he? Especially if you are a smoker. All he can do is try to get you to quit. Well meaning, but half the time he all he succeeds in doing is getting you to want to smoke. He feels an obligation, though.
This is often the way a Christian feels when it comes to the gospel. When we realize the depth of our forgiveness, the amount of sin forgiven by Christ, we can’t help but want to preach this gospel to others. Or course, the others may feel like they just want to smoke a cigarette. Many don’t come to believe. They resist. We can’t help that. All we ca do is proclaim the gospel. We know our obligation. To be sure we can tailor it to our audience. We can put it in foolish terms for the foolish, we can dress it in the robes of philosophy for the wise. But when we know how great our forgiveness is, we can’t help but let others know the joy.