[Natural Law]

"God will not reward those who take his gift of salvation and sit on it instead of using it."

Both Grace and sin require human cooperation. Unlike Grace, sin can be cooperated with by doing nothing.

"For the Son of Man is going to come in his Father’s glory with his angels, and then he will reward each person according to what they have done." - Matthew 16:24

[Dr. Eckleburg]

Your definition of grace is wrong. Grace is defined as God's unmerited favor. Read your Bible.