What does 2 Corinthians 4:12 mean?

This summarizes what Paul has written in the previous verses. Death is at work in him and his co-workers for the gospel of Jesus. For Christ's sake, they are regularly afflicted, persecuted, perplexed, and struck down. They live on the verge of death in fragile human bodies. On the other hand, God has kept them, against serious opposition, from being crushed, from total despair, from being abandoned, and from being destroyed. The fact that they keep going is evidence that God's power is working through them and not their own.

Some among the Corinthians have accused Paul of being a false apostle because he experiences so much suffering. His response is that the life of Christ is at work in the Corinthians exactly because he has been willing to keep going through that suffering, enabled by the power of God. The implication is that instead of challenging his credentials, the Corinthians should be grateful to God for what Paul has endured to bring them life through the gospel of Jesus.