If the Christians in Rome were filled with all knowledge and able to instruct one another, why did Paul find it necessary to write his letter by way of reminder?

The Greek verb used here, nouthetein, translated as instruct actually indicates friendly (Acts 20:31), familial (1 Corinthians 4:14, 2 Thessalonians 3:15) directions and guidelines (Colossians 1:28, Colossians 3:16, 1 Thessalonians 5:14). Paul is well aware of the fact that the Christians in Rome are quite competent to instruct each other. Yet if this is the case and the readers already know all these things and are full of goodness, then it may appear as if it was not necessary for Paul to write the preceding section (Romans 12:1–15:13) at all.

Why then not simply remain silent? Because he wants to strengthen them in that which is good. Moreover, the subject matter lies close to his heart: But on some points I have written to you very boldly by way of reminder, because of the grace given me by God (Romans 15:15). The boldness in his writing does not refer to his particular style, because it is not aggressive or severe. Rather, it concerns the very fact that he is writing about these topics. Paul dared (tolmeros) to write here concerning things that his readers already knew and could manage quite well, yet he did so because it occupied him very much too—not that it necessarily was his favourite or preferred theme, but it just happened to be the one given him by God’s grace. Because of this grace dared to admonish the readers about their attitude toward Israel (Romans 11:1–36) and to criticize them about their attitude toward the weak in the faith (Romans 14:1–23). In all of this Paul was moved by God’s grace.¹