2 Corinthians 12:9
New International Version (NIV)
9 But he said to me, “My grace is sufficient for you, for my power is made perfect in weakness.” Therefore I will boast all the more gladly about my weaknesses, so that Christ’s power may rest on me.
This is Paul writing to the people of Corinth. He had asked God three times to "remove the thorn in his flesh," which we are never told what it is specifically. God did not do his bidding. So Paul concluded that he was supposed to learn from his affliction.
This thorn must have been something chronic and debilitating at times because it sometimes kept Paul from his ministry work.
In questioning why God did not answer his prayer for healing Paul is told that God's grace is sufficient for him and that God's power is made perfect in Paul's weakness. In not receiving healing from God, Paul received things which are much greater -- a stronger character, humility, and a greater capacity for empathizing with others who are suffering.
If God had healed Paul, Paul may not have learned to grow beyond his pain and may not have been such an inspiration to others. Even though Paul was not cured as he had asked, he still continued to believe in God and to preach his word wherever he went. What a greater testimony to God's greatness could he give?
Paul was living proof that courageous faith and holy living do not ensure instant healing of physical ailments.
It also reminds us that our spiritual condition is more important than our physical condition, and that we do not need to be physically healthy in order to be spiritually healthy.
Therefore, Paul boasted in his weakness because he knew that in his weakness he had to rely more on God and that made him all the stronger through God's strength. We must learn to rely on God's strength rather than our own talents, energy, or efforts. Our weakness not only helps develop our Christian faith, it also deepens our worship because we admit that we need to rely on God instead of ourselves.
We should take courage from the fact that God's power can be displayed through weak people. Trying to rely on our own power can lead to pride, whereas relying on God's power can only make us better than we could ever be on our own.