But if it is by grace, it is no longer on the basis of works; otherwise grace would no longer be grace.
Romans 11:6 ESV

Grace is unobligated favor. If being chosen for a role in God’s kingdom could be purchased through deeds or donations (such as Catholic indulgences or purchased offices), then God would be obligated to respond to the required payment.

This doesn’t mean that one person can’t be chosen because he is more capable or more deserving of a role than someone else. It only means that any relevant capability or quality doesn’t obligate God’s choice. He is free to choose a person or a people who is completely unqualified and undeserving by human standards, as he did in the case of Israel, who were chosen solely because of God’s promise to Abraham, not because they had any special merit.