"It is not possible to do works of supererogation. We cannot do what God requires, let alone do more."
This is a statment that has a lot of truth to it. This is part of the thought that underlies the saying that some in the Orthodox Church use: "Even akrevia (strict application of the canons) is a kind of economia (mercifully "loose" application of the canons.)"
In the Orthodox world, often akrevia and economia are talked of as being polar opposites. But this really isn't true, since even the strictest guidelines of prayer, ascetic struggle, and moral behavior leave us far short of the standard of perfection that is indicated by the author's statement that "we are commanded to love God with all of our heart, soul, mind and strength, and our neighbors as ourselves."
On the other hand, Christ's command to "be ye perfect" is not an idle one. We are not the helpless creatures of total depravity that Calvinism describes. We can become saints here on earth, although the vast majority of us will have to wait until the next life to reach the kind of theosis that the great saints achieved here on earth.