Every word of God is pure: he is a shield unto them that put their trust in him.
Hence, (Proverbs 30:4) we must rest in the divinely revealed doctrine, which is pure and saving to all who trust in its Almighty Giver and Object.
Every word of God (is) pure - literally, melted in the fire like silver, so as to be purified of all dross of human error, impurity, deceit, or superfluity. The Word of God supplies the defect of human understanding which Agur complained of (Proverbs 30:2-4).
He is a shield - by His Word and His Spirit foiling the thrusts of Satan (Matthew 4:4; Matthew 4:7; Matthew 4:10).