Chapter 9.5

v.The will of man is made perfectly and immutably free to do good alone in the state of glory only.

According to the Augustinian/Bostonian grammar of sin and grace, heaven is a condition of non posse peccare (not able to sin). Unlike Eden, a condition of probation, in which it was possible to sin and not to sin (posse peccare and posse non peccare), heaven alone finds the regenerate Christian in a condition where sin will be impossible. Nor will this condition be one of constraint: the will in heaven only desires the good and never the evil. Heaven is a condition from which the people of God cannot fall. There is an immutability to this condition. 

It is difficult to fully imagine a condition in which there exists no inclination whatsoever to do anything wicked or evil but the Scriptures hold this out as a reality for God's children - "we shall sin no more" for nothing impure can enter (Rev. 21:27); sin and everything that defiles lies "outside" (Rev. 22:15). 

It is not a biblical world-view to imagine that true freedom must involve the ability to choose the evil inclination. In heaven, we shall be wholly free, but unable to sin. Our wills will voluntarily choose the good.