Paul persecuted all believers that he crossed paths with, starting in Jerusalem, but he was on his way to Damascus, to persecute the church there too - so it wasn't just Jerusalem. If Jesus had not literally stopped him in his tracks, he would most likely have persecuted as many believers as he could, between the two cities.
Matthew 16:18 " ... upon this rock I will build My church." Jesus was prophesying that His universal church, starting then until He returns, would be built on the "rock" that He is the Christ, the Son of the Living God. Yes, the Old Testament saints were redeemed, but Jesus' New Testament church began when He arrived on the scene.
In the Bible, the word "church" is used legitimately in several ways. The context determines which way it is used.
1. A local church, such as a church in a home, of which there were many.
2. A church in a city, such as the church in Ephesus, or the church in Corinth, which was comprised of possibly hundreds of home churches throughout each city.
3. The universal church, which includes all believers, worldwide - millions have gone to heaven, but we are still here.
4. I believe there may be a Biblical reference to the "ekklesia" in the Old Testament, which is translated "church", but that was quite different than the New Testament church, which Jesus founded.
5. Finally, today many call a building used for Christians to meet, a church. I don't think it was ever used that way in the Bible.
So, looking at the context given, we can determine which meaning is referred to.