"What was this good news that Paul preached? It was the news about Jesus of Nazareth. It was the news of the incarnation, the atonement, and the kingdom--the cradle, the cross, and the crown--of the Son of God. It was the news of how God 'glorified his servant Jesus' by making Him Christ, the world's long-awaited 'Prince and... Saviour'. It was the news of how God made His Son Man; and how, as Man, God made Him Priest, and Prophet, and King; and how, as Priest, God also made Him a sacrifice for sins; and how, as Prophet, God also made Him a Lawgiver to His people; and how, as King, God has also made Him Judge of all the world, and given Him prerogatives which in the Old Testament are exclusively Jehovah's own--namely, to reign till every knee bows before Him, and to save all who call on His name. In short, the good news was just this: that God has executed His eternal intention of glorifying His Son by exalting Him as a great Saviour for great sinners.
"Such was the gospel which Paul was sent to preach. It was a message of some complexity, needing to be learned before it could be lived by, and understood before it could be applied. It needed, therefore, to be taught. Hence Paul, as a preacher of it, had to become a teacher. He saw this as part of his caling; he speaks of the 'the gospel: whereunto I am appointed a preacher... and a teacher' (2 Tim. 1:10). And he tells us that teaching was basic to his evangelistic practice; he speaks of 'Christ... whom we preach... teaching every man in all wisdom' (Col. 1:28). In both texts the reference to teaching is explanatory of the reference to preaching. In other words: it is by teaching that the gospel preacher fulfils his ministry. To teach the gospel is his first responsibility: to reduce it to its simplest essentials, to analyse it point by point, to fix its meaning by positive and negative definition, to show how each part of the message links up with the rest--and to go on explaining it till he is quite sure that his listeners have grasped it. And therefore when Paul preached the gospel, formally or informally, in the synagogue or in the streets, to Jews or to Gentiles, to a crowd or to one man, what he did was to teach--engaging attention, capturing interest, setting out the facts, explaining their significance, solving difficulties, answering objections, and showing how the message bears on life. Luke's regular way of describing Paul's evangelistic ministry is to say that he disputed (Acts 9:29), or reasoned (dialegomai: RSV renders 'argued')(Acts 17:2, 17, RV, 18:4; 19:8f., RV, 24:25), or taught (Acts 18:11; 28:31), or persuaded (i.e., sought to carry his hearers' judgments)(Acts 18:4; 19:8, 26; 28:23; cf. 26:28). and Paul himself refers to his ministry among the Gentiles as primarily a task of instruction: 'unto me... was this grace given, to preach unto the Gentiles the unsearchable riches of Christ; and to make all men see what is the dispensation of the mystery...' (Eph. 3:8, RV). Clearly, in Paul's view, his first and fundamental job as a preacher of the gospel was to communicate knowledge--to get gospel truth fixed in men's minds. To him, teaching the truth was the basic evangelistic activity; to him, therefore, the only right method of evangelism was the teaching method."
J.I. Packer, Evangelism and the Sovereignty of God (Downers Grove, IL: IVP), pp. 41, 47-49.