From Andrew A. Bonar’s book, “Memoir and Remains of the Rev. Robert Murray M’Cheyne”, Robert Murray M’Cheyne states:
The whole Bible, and the whole of experience, bear witness that by nature we are ignorant of the truth. No doubt there are many truths which an unconverted man does know. He may know the truths of mathematics and arithmetic – he may know many of the common everyday truths; but still it cannot be said that an unconverted man knows the truth, for Christ is the truth. Christ may be called the key-stone of the arch of truth. Take away the key-stone of an arch, and the whole becomes a heap of rubbish. The very same stones may be there, but they are all fallen, smothered, and confused – without order – without end. Just so – take Christ away, and the whole arch of truth may be there; but they are all fallen – without coherence – without order – without end. Christ may be called the sum of the system of truth. Take away the sun out of our system, and every planet would rush into confusion. The very same planets would be there; but their conflicting forces would draw them hither and thither, orb dashing against orb in endless perplexity. Just so – take Christ away, and the whole system of truth rushes into confusion. The same truths may be in the mind, but all conflicting and jarring in inextricable mazes; for “the path of the wicked is as darkness; they know not at what they stumble.” But let Christ be revealed to an unconverted soul – let it not be merely a man speaking about Christ unto him – but let the Spirit of God reveal him, and there is revealed, not a truth, but the truth. You put the key-stone into the arch of truth; you restore the sun to the center of the system. All truth becomes orderly and serviceable in that mind.