What does 1 Corinthians 1:25 mean?

This verse must be read carefully and in the context of this chapter to be understood correctly. Paul is not saying that God contains any foolishness or weakness, at all. Instead, he has taught why so many people reject the idea of a crucified Christ as weak and foolish. Paul uses the Greek word mōron, which implies precisely what it appears to in English: something stupid, idiotic, or silly. From a godless perspective, it's stupid to think God would sacrifice His only Son to pay the penalty for human sin. Pagan religions, in particular, would call a deity "weak" if it could be killed on a cross like any common criminal or political dissenter.

Those called by God to believe, however, recognize God's foolishness in giving His Son as the greatest of all wisdom. They see the incredible strength required for Christ to remain on the cross and die instead of exercising His power over those who would kill Him.

It's also important to remember that Paul was well-educated (Philippians 3:4–8), experienced (Acts 17:17), tested (Acts 17:11) and frequently encouraged others to use their God-given minds (Colossians 2:8; 2 Timothy 2:15). At no point does the Bible embrace ignorance or sneer at the importance of reason. What Scripture does, clearly, is distinguish between knowing about God (James 2:19) and having a trusting faith in God (Matthew 18:3).