Actual grace, for example, opens the eyes of the sinner that he might realize the malice of his actions. In the same way it sheds light on the mind of the sincere inquirer of the truth. Then after it has illumined the mind, actual grace acts directly on the will to accomplish the good action. It does this by exciting good desires in the will, and by actually conferring on the will the power and courage to perform what it sees is good. When St. Paul preached to the Church at Corinth, and Apollo tended it, the neophyte Christians still had to make the decision to embrace the new religion and live by it.
Human nature is like an intricate steam engine made up of many well-adjusted parts.
The engine can give motion to all these parts, but not until the fire is kindled and the steam rushes through the cylinders to start and keep in motion every piston and wheel. Man with all his wonderful organization and power is dead and useless unless the grace of God illumines and inspires him. The mind of man, even without any help from actual grace, can arrive at certain truths about God. The human mind has a love for truth and it has great powers of investigation. Some pagan philosophers recognized the author of nature as God and even knew something about His essence and perfections.