On that journey as I drew near to Damascus, about noon a great light from the sky suddenly shone around me. I fell to the ground and heard a voice saying to me, 'Saul, Saul, why are you persecuting me?' I replied, 'Who are you, sir?' And he said to me, 'I am Jesus the Nazorean whom you are persecuting.'
The witnessing of Paul has given us a testimony of Jesus' love. Saul in his on way was bent on persecuting Christians. In a sense he is worst than us in that regard. But in his zeal to destroy the kingdom on God. God forgave him, and chose him to bring the Good News to all the people.
How come Saul and not someone else? Maybe it was in the way Saul had zeal to destroy, and God in his mercy used that to convert Saul to do his will so that zeal will be used for God' kingdom. We do not know the mind of God and why he chooses sinners to bring witness to his kingdom.
But when we think about how a heart is converted and transformed from an encounter with Christ. The power of that conversion alone is God's proof of his love for us and that individual. Just as Jesus said, those who are well need no physician. Those who sin need one. That's why he calls them out to transform them, to witness for those whose faith is weak so it will be strengthened. Just as those with a strong faith do not need this witness, they believe without a shadow of a doubt.