If grace does everything for them, what reason would he have for recompensing them?
The only other attempt online to answer these questions, that I've found, doesn't answer this one. It simply confesses to not understand the question. I agree! But, I can see a possible line of thought.
Shelley is right to say that God's grace does everything for believers - grace is all we need. So the question rises why would God recompense us. Recompense meaning returning in kind, payment for service. I can only conclude that Shelley observed a kind of "christianity" where it seemed that God was subservient to humanity, rewarding our good deeds, such as they might be.
Such "christianity" is a dire perversion of the truth. God is not our debtor and God does not reward good works. Many think that this is exactly what Christianity is - that the good people do good stuff and God rewards them with blessings and heaven. This is not true. Humanity is not good. We deserve no good thing from God.
Narnia author C.S. Lewis noted in the 20th Century that grace was the distinctive that marked out Christianity from all other "religious" systems. Bono noted that everything else is Karma but Jesus is about grace. Everything else is deeds and rewards, but Christ gives freely. To attempt to gain favour with God by good deeds would be to opt out of his grace - to refuse it. God is not interested in recompensing us for devotion. Living that way exalts us and our deeds, rather than God and his grace.
And how does this grace come to us? It comes through the death of Jesus Christ in our place, enabling God to show pour out abundant grace upon us. And it comes in the form of God himself. The gift of the greatest treasure of all. God gives us himself. Not by our efforts, but of himself - for his own sake, not because we deserved him.
Abraham Piper expresses this well in his song:
The Gift you Give (music P. Carlson)
From you our life arrives
And into you departs.
Then we begin to live.
You are the life of lives.
You are the heart of hearts.
You are the gift you give.
You are the soul of every soul
That comes to you to be made whole.
You are the one for everyone
Who longs for life—life in your love—
To finally see the beauty of
The gift you give of Christ the Son.
To suggest that God recompenses believers would be to misunderstand the very nature of grace - the gift of God to undeserving people who believe. And not because they summoned up enough belief in themselves, for even belief is a gift.
If Shelley was put off Christ by a works based twisting of the truth then the tragedy is that he rejected a shadow of the real thing. Next - If he is all-powerful, how offend him, how resist him? A question straight out of scripture itself...