What Makes Saint John Vaney So Powerful?

I think the devil hated him so much because he was the instrument of mercy, of God's mercy. His intimate union with Christ in prayer, his love for the Lord and the Eucharist,. They said that his eyes just changed when he would hold the host up as a celebrating Mass. I always think of him as like this very simple priest who did unbelievable things. And also it's something that you always need reminders to get out of God's way.