Disagreeing with a Saint

I came across this quote from St. Isaac of Nineveh today:

"Be at peace with your own soul, then heaven and earth will be at peace with you."

I've read some of St. Isaac's writings before, and am generally of the same mindset, but this time I'm not so sure.

It's easy to see how you will be at peace with the Universe in general if you are at peace with your own soul. However, there are plenty of mortals here on earth who have their own agendas, and who are definitely not in harmony with you no matter how certain you are of your own path.

Maybe some of my philosophically-minded friends will add their thoughts!