John Calvin Commentary Psalms 18:30

"As for God, his way is perfect: The word of Jehovah is tried; He is a shield unto all them that take refuge in him." — Psalms 18:30 (ASV)

The way of God is perfect. The phrase, The way of God, is not understood here as His revealed will, but as His method of dealing with His people. Therefore, the meaning is that God never disappoints or deceives His servants, nor forsakes them in their time of need (as can be the case with people who only help their dependents when it serves their own advantage), but faithfully defends and sustains those whom He has once taken under His protection.

But we will never have any nearness to God unless He first comes near to us by His word. For this reason, David, after asserting that God earnestly aids His people, also adds that His word is purified. Therefore, let us rest assured that God will indeed show Himself upright towards us, since He has promised to be the guardian and protector of our welfare, and His promise is certain and infallible truth.

That by the word we are not here to understand the commandments, but rather the promises of God, can be easily gathered from the following clause, where it says, He is a shield to all those who trust in him. Indeed, it seems a common commendation to say that the word of God is pure and without any mixture of fraud and deceit, like silver that is well refined and purified from all its dross.

But our unbelief is the reason why God, so to speak, is compelled to use such a simile to commend and lead us to form exalted conceptions of the steadfastness and certainty of His promises. For whenever the outcome does not meet our expectation, we are naturally very prone to immediately begin to entertain unholy and distrustful thoughts about the word of God. For a further explanation of these words, we refer our readers to our remarks on Psalm 12:6.