Albert Barnes Commentary 1 Corinthians 4:20

"For the kingdom of God is not in word, but in power." — 1 Corinthians 4:20 (ASV)

For the kingdom of God. The reign of God in the church; meaning here, probably, the power or authority that was to be exercised in the government and discipline of the church. Or it may refer to the manner in which the church had been established. "It has not been set up by empty boasting; by pompous pretensions; by confident assertions. Such empty boasts would do little in the great work of founding, governing, and preserving the church; and unless men have some higher powers than this, they are not qualified to be religious teachers and guides."

But in power.

1. In the miraculous power by which the church was established—the power of the Saviour and of the apostles in working miracles.
2. In the power of the Holy Ghost in the gift of tongues, and in his influence on the heart in converting men.
3. In the continual power that is necessary to protect, defend, and govern the church. Unless teachers showed that they had such power, they were not qualified for their office.