Objection 1. It seems that by natural reason we cannot know God in this life. For Boethius says (De Consol. v) that "reason does not grasp simple form." But God is a supremely simple form, as was shown above (Question 3, Article 7). Therefore natural reason cannot attain to know Him.
Objection 2. Further, the soul understands nothing by natural reason without the use of the imagination. But we cannot have an imagination of God, Who is incorporeal. Therefore we cannot know God by natural knowledge.
Objection 3. Further, the knowledge of natural reason belongs to both good and evil, inasmuch as they have a common nature. But the knowledge of God belongs only to the good; for Augustine says (De Trin. i): "The weak eye of the human mind is not fixed on that excellent light unless purified by the justice of faith." Therefore God cannot be known by natural reason.
On the contrary, It is written (Romans 1:19), "That which is known of God," namely, what can be known of God by natural reason, "is manifest in them."
I answer that, Our natural knowledge begins from sense. Hence our natural knowledge can go as far as it can be led by sensible things. But our mind cannot be led by sense so far as to see the essence of God; because the sensible effects of God do not equal the power of God as their cause. Hence from the knowledge of sensible things the whole power of God cannot be known; nor therefore can His essence be seen. But because they are His effects and depend on their cause, we can be led from them so far as to know of God "whether He exists," and to know of Him what must necessarily belong to Him, as the first cause of all things, exceeding all things caused by Him.
Hence we know that His relationship with creatures so far as to be the cause of them all; also that creatures differ from Him, inasmuch as He is not in any way part of what is caused by Him; and that creatures are not removed from Him by reason of any defect on His part, but because He superexceeds them all.
Reply to Objection 1. Reason cannot reach up to simple form, so as to know "what it is"; but it can know "whether it is."
Reply to Objection 2. God is known by natural knowledge through the images of His effects.
Reply to Objection 3. As the knowledge of God's essence is by grace, it belongs only to the good; but the knowledge of Him by natural reason can belong to both good and bad; and hence Augustine says (Retract. i), retracting what he had said before: "I do not approve what I said in prayer, 'God who willest that only the pure should know truth.' For it can be answered that many who are not pure can know many truths," i.e. by natural reason.
Source: Summa Theologica (New Advent)