The imperial guard
(Philippians 1:13) was a group of some 9,000 soldiers.1 It became widely known among them that Paul was under arrest, not for some heinous crime, but simply for the sake of his faith in Christ. The way this would have spread is surprisingly simple.
Paul had a soldier watching him around the clock, and almost certainly, that soldier was chained to Paul. And when his shift was over, another soldier took his place. So over the course of those two years, Paul saw a lot of soldiers and a lot of soldiers spent many hours with him. They saw him kneel before the Father in prayer for the Philippian believers and others. They witnessed him pouring over and meditating on God’s Word. They heard him dictate his letters. They listened to his personal conversations with his friends, and heard him persuade others of the truth of the gospel as he spoke of God’s undeserved grace, about how he sent Jesus for sinners, and about the hope of the resurrection
All this gave Paul ample opportunity to try to win these soldiers for Christ! They no doubt heard the gospel call: the call for anyone who is weary and burdened to come and find rest in Jesus. The call that anyone who believes in Jesus will be saved, no matter what they have done. No matter how far they have fallen, forgiveness is theirs. Christ's blood was shed once and for all!
13 so that it has become known throughout the whole imperial guard and to all the rest that my imprisonment is for Christ.