Much knowledge may be gained from nature. Paul plainly states that all rational men are without excuse not to believe in the eternity and power of God. It does not take grace to conclude logically that that which has design manifests a Designer of great wisdom and power, so the wicked cannot plead ignorance for their atheism and idolatry.
Nature, however, is not sufficient to explain to us the wonderful works of God in the salvation of sinners. Men are blameworthy not to acknowledge an eternal and powerful God, but they cannot be blamed for not understanding the way of salvation if they have never had it explained to them. The eunuch made that plea: “How can I [understand], except some man should guide me?” If man were by his logic able to understand how men could be saved, then men might argue that we might have been God’s counselor in the matter of salvation; but Paul asked, “For who hath known the mind of the Lord? or who hath been his counselor?” (Rom. 11.34). No man could advise God in how to save sinners because no man knew how sinners could be saved; and, indeed, no man by nature had an interest in the subject.
If men were able by nature to understand how it is that men could be saved, then it would not be a mystery; but Paul called it a mystery. “Great is the mystery of godliness: God was manifest in the flesh” (1 Tm. 3.16). Men were to be saved by the second Person of the Godhead taking unto Himself a pure and complete human nature, keeping the exacting rigors of the Mosaic law, and suffering the cruel death of crucifixion. No man ever would have arrived at that scheme of salvation.
The way of salvation is revealed fully in the Scriptures, and only in the Scriptures. “All scripture is given by inspiration of God, and is profitable for doctrine” (1 Tm. 3.16). Doctrine is teaching. The Scriptures are profitable for the teaching of men, and in what greater subject do they instruct us than how sinners are saved? Nature teaches us that there is a God and somewhat of His nature, but man never could have learned about salvation from nature. Paul spoke of his wonderful “knowledge in the mystery of Christ” that God had revealed to him. The Christ was the anointed Savior, God manifest in the flesh, who saved His people from their sins. Paul said that this mystery “in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit.” Nature could not reveal it, but God, through the inspiration of holy men to write the Scriptures, has revealed it to us.