Paul says repeatedly, he will boast only of Jesus and the cross. That doesn't mean he is neglecting all the education he got.

As a student of Gamaliel, Paul would've learned philosophy and way of presenting arguments. Paul is displaying that learning here to present the Gospel to the Jews in Rome.

Knowing that his audience, Jews, consider prophets as the final authority, Paul says the gospel was promised through the prophets.

When you witness to your friends and colleagues, remember the technique of Paul. Be mindful of your audience and find a saying or a story that will form as the stepping stone to your witness.

Having built a bridge to his audience, now Paul places Jesus and contrasts him with other respected men.

Here is the template for witnessing: understand your audience, find a common ground, juxtapose Christ, and rely on trinity. You'll be a fruitful witness.

Go and tell about Jesus. The world is waiting to hear from you.