Of course the kingdom of God is in word, if we are talking about God’s Word, and God’s word is powerful. But the apostle Paul is not talking here about God’s word.
The kingdom of God means the rule of God, the reign of God in hearts. Its presence is not detected by what you claim, so much as by the power which is at work in you. If God is reigning in your heart, his power will be operating to keep you close to himself, to make you a concerned, feelingful Christian man or woman.