The preaching of the Word lets us know. That we are saved by grace though faith, and that the foolishness of preaching is only foolish to those who won't come to the cross and accept that Jesus Christ died, shed his blood for the sins of the whole world and that God raised him the third day victorous over death, hell and the grave.
Revelation 3:20 Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me.

1 Corinthians 1:18