I have seen many question and views here on the apostleship of St. Paul. Questions like this, this, this and Most recently this. All seems to basically ask why Paul was chosen as apostle or whether he was really an apostlen. Are we unaware of what God foresaw in appointing Paul as apostle?
One possible explanation that God might have done this could be:
There would be no meaning and no purpose fulfilled by Jesus dying on cross, if the events in New Testament are not LINKED to the Old Testament. And who will be in a better position to link these two Testaments, than a learned Jews Scripture Scholar, that was St Paul. St. Paul was in the best and ideal position to know and understand clearly the Plan that God had since the fall of the first men.
So possibly, it was God’s divine plan to mould St. Paul into what he was before his conversion and impart in him, the in depth knowledge of Jewish Scripture and then convert him so forcibly and not through the preaching from disciples of Jesus but by His own power by striking him down when he was travelling to prosecute more of Christians population.
Whatever that he has written in the New testament would not have been possible for any other disciples of Jesus and there would have been no clear understanding about the purpose of Jesus coming in such a manner and dying a kind of death despised very much at that period of time.
Jesus disciples were master in the message taught to them by Jesus but were lacking in in-depth knowledge of Jewish scripture as they were just ordinary people prior to hearing the message from Jesus. So it was like a double edged sword-- Paul with a thorough knowledge of Old Testament and Jesus’ original disciples with thorough knowledge of message from Jesus.
So God’s plans are truly marvellous and at time we are unable to understand them and only start to understand them as the time passes.
PS: I have provided an answer to my own question but some body can improve this answer or post another answer.
We do ourselves and all Christians a fundamental disservice whenever we give what sounds like a definitve answer to any question starting "Why did God...". God's ways are not our ways, and his thoughts are higher than our thoughts. The best we can ever hope to say is: "I believe that one reason God did this might have been...". Unless God has told us the answer elsewhere, of course.