1 Corinthians 2:4

2:4 My conversation and my preaching were not with persuasive words of wisdom, but with a demonstration of the Spirit and of power,

1 Corinthians 15:14

15:14 And if Christ has not been raised, then our preaching is futile and your faith is empty.

1 Corinthians 1:21

1:21 For since in the wisdom of God the world by its wisdom did not know God, God was pleased to save those who believe by the foolishness of preaching.