“Hence is the stability of grace, and perseverance of the saints; it is founded upon his unchangeableness. Not that they are so, though truly sanctified, if they and their graces were left to their own management; no, it is he who not only gives that rich portion to those he adopts to be his children, but keeps it for them, and them in the possession of it; He maintains the lot of our inheritance, Psal. xvi. 6. And to build that persuasion of perseverance upon his truth and power engaged in it is no presumption, yea, it is high dishonour to him to question it.
“But when nature is set to judge of grace, it must speak according to itself; and, therefore, very unsuitably to that which it speaks of. Natural wits apprehend not the spiritual tenor of the Covenant of Grace, but model it to their own principles, and quite disguise it; and they think of nothing but their resolves and moral purposes: or they take up with a confused notion of grace; they imagine it put into their own hands, to keep or lose it, and will not stoop to a continual dependence on the strength of another; rather choosing that game of hazard, though it is certain loss and undoing, to do for themselves.
“But the humble believer is otherwise taught; he 'hath not so learned Christ.' He sees himself beset with enemies without, and buckled to a treacherous heart within, that will betray him to them; and he dare no more trust himself, to himself, than to his most professed enemies. Thus it ought to be, and the more the heart is brought to this humble petitioning for that ability, and strengthening, and perfecting, from God, the more shall it ﬁnd both stability and peace, from the assurance of that stability.
“And certainly, the more the Christian is acquainted with himself, the more will he go out of himself for his perfecting and establishing. He ﬁnds, that when he thinks to go forward, he is driven backward, and sin gets hold of him, oftentimes, when he thought to have smitten it. He finds that miserable inconstancy of his heart in spiritual things, the vanishing of his purposes and breaking off of his thoughts, that they usually die ere they be brought forth: so that when he hath thought, 'I will pray more reverently, and set myself to behold God when I speak to him, and watch more over my heart, that it ﬂy not out and leave me:' possibly the ﬁrst time he sets to it, thinking to be master of his intention, he ﬁnds himself more scattered, and disordered, and dead, than at any other time. When he hath conceived thoughts of humility and self-abasement, and thinks, 'Now I am down, and laid low within myself, to rise and look big no more;' yet some vain fancy creeps in anon, and encourages him, and raises him up to his old estate; so that in this plight, had he not higher strength to look at, he would sit down and give over all, as utterly hopeless of ever attaining to his journey’s end.
“But when he considers whose work that is within him, even these small beginnings of desires, he is encouraged by the greatness of the work, not to despise and despair of the small appearance of it in its beginning; not to despise the day of small things, Zech. iv. 10; and knowing that it is not by any power nor might, but by his Spirit, that it shall be accomplished, he lays held on that word, Job viii. 7, Though thy beginning be small, yet thy latter and shall greatly increase.”
Robert Leighton, Commentary on 1 Peter 5:10