Here the kingdom of God
refers to God’s acts of ruling (and not so much to the domain of his rule). Whereas Paul’s arrogant opponents can only rely on talk,
Paul can prove his alliance to God’s kingdom with power.
As an apostle, Paul had received power from the Spirit to perform miracles (1 Corinthians 2:4–5; 2 Corinthians 11:11). At the time when he was writing this letter, his fame was spreading throughout Ephesus due to the great miracles that God was performing through him (Acts 19:11–17).
20 For the kingdom of God does not consist in talk but in power.