In 1 Corinthians 1:10–17, Paul states that the Corinthian believers wrongly tried to use their personal associations with noted Christians to gain church authority. The rest of the chapter explains that such behavior is the worldly way of doing things and incongruous with the power of salvation available only in the cross.
Verse 18, we have seen, reminds us that the word of the cross is the power of God to those who are being saved—believers—but folly to the perishing—the world, which is passing away (1 John 2:17). Why is it foolishness to the world? Because it proves that God effects salvation according to His terms, not our own. Paul argues this in a series of points that begin with his quotation of Isaiah 29:14 in 1 Corinthians 1:19. Isaiah 29 predicts the Assyrian invasion of Judah in the eighth century B.C. and that Judah would be saved by the Lord’s direct intervention and not by human military power. This prediction was fulfilled when the angel of the Lord defeated the Assyrian army, forcing Assyria to withdraw (chs. 36–37). Herodotus, an ancient Greek historian, reports that a plague of disease-riddled mice came upon the Assyrian army at that time. So, the angel of the Lord may have used mice to turn Assyria away. In any case, the point in Isaiah 29 is that God would unilaterally save His people in an unexpected way that would cause them to marvel.
That is how God accomplished our final salvation through the cross, which is why Paul quotes Isaiah. Redemption comes in an unexpected, even seemingly foolish, manner—only through the cross (1 Cor. 1:19–20). Most Jews, not knowing the Scriptures, expected a conquering Messiah, not one hung on a tree, cursed in our place by God (see Deut. 21:22–23). They looked for a mighty sign such as the overthrow of the Roman Empire. The Greeks, or gentiles, expected salvation by wisdom, by knowing the secret, hidden truth about the world or the path to follow to earn salvation (1 Cor. 1:22). Nothing could have been more foolish to them than that we are saved by the God-man who dies—according to His human nature—on the cross.
Against worldly expectations, God sovereignly achieved salvation through Christ on the cross. Thus, “the foolishness of God is wiser than men, and the weakness of God is stronger than men” (v. 25). The power of salvation comes through apparent weakness. The world’s way of exercising strength via personal associations opposes the way of the cross, so it is wrong to follow the world’s way in the church.