“This is the voice of our conscience, telling us of the righteousness of God. And since conscience is the perfect interpreter of life, what it tells us is no question, no riddle, no problem, but a fact — the deepest, innermost, surest fact of life: God is righteous.”

"The Righteousness of God" (1916) in The Word of God and the Word of Man (1928) as translated by Douglas Horton; this passage begins with a quote of Isaiah 40:3-5; often quoted alone has been the phrase following it: "Conscience is the perfect interpreter of life."
Context: "The voice of him that crieth in the wilderness, Prepare ye the way of the Lord, make straight in the desert a highway for our God. Every valley shall be exalted, and every mountain and hill shall be made low: and the crooked shall be made straight, and the rough places plain: And the glory of the Lord shall be revealed!" This is the voice of our conscience, telling us of the righteousness of God. And since conscience is the perfect interpreter of life, what it tells us is no question, no riddle, no problem, but a fact — the deepest, innermost, surest fact of life: God is righteous. Our only question is what attitude toward the fact we ought to take.
We shall hardly approach the fact with our critical reason. The reason sees the small and the larger but not the large. It sees the preliminary, but not the final, the derived but not the original, the complex but not the simple. It sees what is human but not what is divine.
We shall hardly be taught this fact by men.