Paul had many roles as an apostle of Jesus Christ and he stresses one of them here, that of an ambassador. This is the second time he has used this concept, the other being 2 Corinthians 5:20: ‘Therefore, we are ambassadors for Christ, God making his appeal through us. We implore you on behalf of Christ, be reconciled to God.’
He describes himself here as an ambassador in chains. He may be saying that his chains are the equivalent of an ambassador’s chains of office that he wore to indicate his authority to speak for his master. Paul regarded these chains as being put round his neck by Jesus Christ; he saw them as badges of honour.
These chains were also a reminder of the price he had paid to be an ambassador of Jesus Christ. In political and civil actions, an ambassador was treated with great respect, even if the countries were not on friendly terms. But the ambassador of the King of kings had been mistreated and despised. He had paid a heavy price for his faithfulness.
Or it could be that Paul is here describing the location where he functioned as an ambassador. Usually ambassadors functioned in palaces or in the presence of the leaders. They operated in the places of power. Yet the Lord Jesus, the universal king, had sent his ambassador to serve him in the lowest place on earth.
Again, Paul may be saying that earthly chains cannot prevent him functioning as an ambassador of Christ. The civil and religious authorities may have tried to silence him, but they were not able to do so. There was nothing they could do, as long as he was alive, to prevent him functioning as Christ’s ambassador.