John Calvin Commentary Acts 9:1

"But Saul, yet breathing threatening and slaughter against the disciples of the Lord, went unto the high priest," — Acts 9:1 (ASV)

And Saul. Luke sets down in this place a noble history, and a history very well worthy of being remembered, concerning the conversion of Paul: how the Lord not only subdued him and made him subject to His commandment when he raged like an untamed beast, but also how He made him a different and new man. But because Luke sets down all things in order, as in a notable work of God, it will be more convenient to follow his text, so that everything worth noting may come in order.

When he says that he was still breathing out threatenings and slaughter, his meaning is that after his hands were once stained with innocent blood, he proceeded with similar cruelty and was always a furious and bloody enemy of the Church, after he had once made that entrance mentioned in connection with the death of Stephen.

For this reason, it was all the more incredible that he could be so suddenly tamed. And since such a cruel wolf was not only turned into a sheep but also took on the nature of a shepherd, the wonderful hand of God showed itself clearly in this.