He rewarded the midwives of Egypt because they refused to kill the male infants of the Hebrew women as Pharaoh had commanded them to do. But the reward was on the same natural level as the good work done. A virtuous man, devoid of grace, is limited even in his own natural sphere. There are some natural virtues, for example, which he cannot acquire at all, or only with great difficulty, or for a short time. Still more is he incapable of reaching that high standard of morality proposed by Christ in the Sermon on the Mount.
Grace is necessary.
In fact, so necessary is actual grace that even when a person is already in the state of grace he needs this transient help each time he performs a good deed. I am the vine, you are the branches. No one can ever stand alone. He is always dependent on God's grace whether it be at the end of a long life of faithful service, or at the moment of taking the first step, whether it be to rise from the sleep of sin and be brought back to God, or to persevere in a holy life until death. It might seem from what has been said that grace does all the work in our sanctification.