The Basis of Inward Quietness is Faith

The basis of this remarkable and interesting state of mind [that is, inward quietness] is FAITH. 

In the first place, it is faith, operating by love. That is to say, a faith in the character of God, which results in the restoration of love to God. Those, who believe God, love God; those, who believe him much, love him much; those, who believe perfectly, love perfectly. The sequence of love to faith, both in fact and degree, is not a mere matter of arbitrary choice or volition; but may rather be regarded as the result of a permanent and unchangeable law, a law which is true now and true always, which exists on earth and exists every where else. 

And we may add, that those, who love God as they ought to love him, cannot love other things otherwise than they ought to. The love of God in the heart, existing in accordance with the commandment, viz., thou shalt love the Lord thy God with all thy heart, cannot fail to bring every desire, every affection, which has relation either to ourselves or to others, into subjection. Every desire, every affection, every tendency of our nature which is susceptible of a moral character, resumes, from that memorable moment, its true position. And when order is thus restored to the mind, by the reduction of every thing to its proper place, quietness of spirit exists and prevails as a necessary result. It is true it is no common love, which can effect this; and consequently it is no common degree of faith which gives rise to such love. But a grace so eminent as that of true quietness of spirit cannot be expected to exist where faith is weak.

In the second place, the grace of quietness of spirit is sustained by faith in God’s providences; or perhaps we should say more specifically, by faith in God’s presence in his providences. We have already had occasion to refer to this great practical doctrine, that, in the succession of God’s providences, God himself is hidden in the bosom of every event. He is there, although he is not always seen. He is there to watch and control, if he is not there to originate. So that we can truly say, that no event in his providence happens, without bringing God with it, and without laying his hand upon us. The man of faith, therefore, knows, (and he cannot know it without bringing it home to his own case,) that he, who is impatient with events, is impatient with God; he who frets at events frets at God; he, who is not acquiescent in events, is at war with God. In such a position he cannot, he dare not place himself.

— from The Life of Faith, Part 2, Chapter 13.