The following is a quote from The Christian in Complete Armour by William Gurnall. It is rather long, but well worth reading and rereading.

[Why the saint’s strength is laid up in God.]

Reason First. The first reason may be taken from the nature of the saints and their grace. Both are creatures, they and their grace also. Now, ‘it is in the very nature of the creature to depend on God its Maker,’ both for being and operation. Can you conceive an accident to be out of its subject, whiteness out of the wall, or some other subject? It is as impossible that the creature should be, or act without strength from God. This to be, act in and of himself, is so incommunicable a property of the Deity, that he cannot impart it to his creature. God is, and there is none besides him. When God made the world, it is said indeed he ended his work, that is, of creation: he made no new species and kinds of creatures more; but to this day he hath not ended his work of providence: ‘My Father worketh hitherto,’ saith Christ, Jn. v. 17, that is, in preserving and empowering what he hath made with strength to be and act, and therefore he is said to hold our souls in life. Works of art, which man makes, when finished, may stand some time without the workman’s help, as the house, when the carpenter has made it is dead; but God’s works, both of nature and grace, are never off his hand, and therefore as the Father is said to work hitherto for the preservation of the works of nature, so the Son, to whom is committed the work of redemption, he tells us, worketh also. Neither ended he his work when he rose again, any otherwise than his Father did in the work of creation. God made an end of making, so Christ made an end of purchasing mercy, grace, and glory for believers, by once dying; and as God rested at the end of the creation, so he, when he had wrought eternal redemption, and ‘by himself purged our sins, sat down on the right hand of the majesty on high,’ He. i. 3. But he ceaseth not to work by his intercession with God for us, and by his Spirit in us for God, whereby he upholds his saints, their graces, and comforts in life, without which they would run to ruin. Thus we see as grace is a creature, the Christian depends on God for his strength. But further,

Reason Second. The Christian’s grace is not only a creature, but a weak creature, conflicting with enemies stronger than itself, and therefore cannot keep the field without an auxiliary strength from heaven. The weakest goes to the wall, if no succour comes in. Grace in this life is but weak, like a king in the cradle, which gives advantage to Satan to carry on his plots more strongly to the disturbance of this young king’s reign in the soul, yea, he would soon make an end of the war in the ruin of the believer’s grace, did not Heaven take the Christian into protection. It is true indeed, grace, whereever it is, hath a principle in itself that makes it desire and endeavour to preserve itself according to its strength, but being overpowered must perish, except assisted by God, as fire in greenwood, which deads and damps the part kindled, will in time go out, except blown up, or more fire put to that little; so will grace in the heart. God brings his grace into the heart by conquest. Now, as in a conquered city, though some yield and become true subjects to the conqueror, yet others plot how they may shake off this yoke; and therefore it requires the same power to keep, as was to win it at first. The Christian hath an unregenerate part, that is discontented at this new change in the heart, and disdains as much to come under the sweet government of Christ’s sceptre, as the Sodomites that Lot should judge them. What, this fellow, a stranger, control us! And Satan heads this mutinous rout against the Christian, so that if God should not continually reinforce this his new planted colony in the heart, the very natives (I mean corruptions) that are left, would come out of their dens and holes where they lie lurking, and eat up the little grace the holiest on earth hath; it would be as bread to these devourers.

Reason Third. A third demonstration may be taken from the grand design which God propounds to himself in the saint’s salvation; yea, in the transaction of it from first to last. And that is twofold. 1. God would bring his saints to heaven in such a way as might be most expressive of his dear love and mercy to them. 2. He would so express his mercy and love to them, as might rebound back to him in the highest advance of his own glory possible. Now how becoming this is to both, that saints should have all their ability for every step they take in the way to heaven, will soon appear.

1. Design. God would bring his saints to heaven in such a way as might be most expressive of his dear love and mercy to them. This way of communicating strength to saints, gives a double accent to God’s love and mercy.

(1.) It distils a sweetness into all the believer hath or doth, when he finds any comfort in his bosom, any enlargement of heart in duty, any support under temptations, to consider whence came all these, what friend sends them in. They come not from my own cistern, or any creature’s. O it is my God that hath been here, and left his sweet perfume of comfort behind him in my bosom! my God that hath unawares to me filled my sails with the gales of his Spirit, and brought me off the flats of my own deadness, where I lay aground. O, it is his sweet Spirit that held my head, stayed my heart in such an affliction and temptation, or else I had gone away in a fainting fit of unbelief. How can this choose but endear God to a gracious soul? His succours coming so immediately from heaven, which would be lost, if the Christian had any strength to help himself (though this stock of strength came at first from God). Which, think you, speaks more love and condescent: for a prince to give a pension to a favourite, on which he may live by his own care, or for this prince to take the chief care upon himself, and come from day to day to this man’s house, and look into his cupboard and see what provision he hath, what expense he is at, and so constantly to provide for the man from time to time? Possibly some proud spirit that likes to be his own man, or loves his means better than his prince, would prefer the former, but one that is ambitious to have the heart and love of his prince would be ravished with the latter. Thus God doth with his saints. The great God comes and looks into their cupboard, and sees how they are laid in, and sends in accordingly as he finds them. ‘Your heavenly Father knows you have need of these things,’ and you shall have them. He knows you need strength to pray, [to] hear, [to] suffer for him, and, in ipsâ horâ dabitur, ‘in the very hour it will be given.’

(2.) This way of God’s dealing with his saints adds to the fulness and stability of their strength. Were the stock in our own hands, we should soon prove broken merchants. God knows we are but leaking vessels, when fullest we could not hold it long; and therefore to make all sure, he sets us under the streamings forth of his strength, and a leaking vessel under a cock gets what it loseth. Thus we have our leakage supplied continually. This was the provision God made for Israel in the wilderness: He clave the rock, and the rock followed them. They had not only a draught at present, but it ran in a stream after them, so that you hear no more of their complaints for water. This rock was Christ Every believer hath Christ at his back, following him with strength as he goes, for every condition and trial. One flower with the root is worth many in a posie, which though sweet yet do not grow, but wither as we wear them in bosoms. God’s strength as the root keeps lively, without which, though as orient as Adam’s was, it would die.