Father help us to show our zeal for you.
It's often the case but when someone shows a certain passion for something they do it with all their heart. This may be positive or negative. Sometimes people go out of their way because they have a particular like or dislike. In some ways it can change a person's life.
Paul was such a person. In a lot of ways he had the perfect upbringing for someone who would preach Jesus Christ to the world. He was a skilled scholar of Jewish Law. He had the personality to stand up for his beliefs. The problem was that he saw Christ as an intrusion to the law. He went so far as to persecute people who followed Jesus. He wasn't the type of person just to sit by and let things happen. He wanted things to be just as he was taught. But God saw the potential, drive and energy that he had.
All that was needed was to divert that energy towards the right course. As you've probably already come across in physics, there are the laws of energy. Energy is transmuted. It can move things from one direction towards another (such as a tennis ball being hit by a racket) or it can change state such as from motion energy to heat (in a high speed impact or friction) or from light to electricity(in a solar panel). It something looses its energy if becomes inert.
God didn't want Paul (or us) to be inert or apathetic. He wanted us to go out and do things, to use the energy God gave each one of us for something positive. It sounds like a contradiction in terms but apathy can be a very destructive force.
So the first thing we need is energy. We have to try hard, to try our best. Paul likens it to an athlete. We have to train. Sometimes a little each day is better than a huge effort every now and again. But the important thing is that something is done, that we don't just sit back and let others do the work.
Paul had the energy to go out and preach, not just to his normal circle of friends, but to thousands of people in dozens of countries. In some ways he may have been trying to atone for his past mistakes but we must realise the importance of his work. He went at it as good as his ability would allow.
Without Paul, and especially his letters, we would not have the same Christianity that we know today. While some of Paul's letters are hard to understand (even the Apostle Peter says that) we have a grounding for the teachings of Christ. Without his commitment and hard work we would not have such as complete picture of Christ or the workings of God.