The first line of Wilbur's poem, Beasts, reads "Beasts in their major freedom/ Slumber in peace tonight." In an out of print book called TALKS WITH AUTHORS from the early sixties, Wilbur says "I mean various things by that word 'major' but the expression, 'major freedom,' comes from one of the church fathers -- I forget which one. He distinguishes between major freedom and minor freedom and says that the saints enjoy major freedom because their wills are at one with God's will. They don't have to choose anymore. In this poem of mine I attribute that kind of freedom to the animal creation."
I've not been able to find yet which church father says that.