Jesus's cross proves that God has redeemed sinners from our eternal demise. Bearing the fruit of godliness proves that we have accepted that gift.
Human hands held the hammers that drove the nails through Jesus' body. Human governors authorized the crucifixion of Jesus. However, in the light of the cross, we can see that this was a decisive battle in the war of the ages. God's enemies thought they dealt the final blow to defeat God's plan of redemption. They were wrong.
The good news that Christians want to communicate to the world sounds like nonsense to everyone but Christians. Why would God use such an unworldly approach to saving lost souls?
The word of God is powerful. It makes things happen. Paul testifies that the simple, unadorned message of the cross of Jesus reveals both the righteousness and the wrath of God. At the same time, it reveals those who are perishing and those who are saved. Christians can trust the word of God to do the work of God as we reach out to save the lost in the world today.