That a thing may be perfect in its nature, and yet be susceptible of growth or advancement in degree, is, I suppose, a matter of common observation. An oak, when it first rises above the surface of the ground, is so small and weak, that it may be easily trodden under foot; and yet it is as really and truly an oak, as when it subsequently stands forth in the strength and stature of an hundred years. A human being is in his nature as much a human being in the period of infancy, as in the subsequent expansion and growth of manhood. And so consider a man in relation to any intellectual power of the mind, or in relation to any appetite or affection of the mind, and the same view may very properly be taken. A person is a reasoner, for instance; he understands perfectly the principles and process of reasoning, and he may be able to apply the principles and process perfectly in a given case, and yet under the favorable influence of the law of Habit, he may much increase the promptness and facility, and consequent perfection, in the operations of this mental faculty. Again, an intemperate man may become perfectly temperate, and yet we all know the general fact, that one, who is thus entirely reformed from intemperance, is more likely to be overcome by temptation in the earlier periods of his reformation, than when subsequently the temperate principle has acquired growth and strength.
And we may not only say in general terms, that there may be a growth in perfection, but may assert further, that the thing which is most perfect, if it be susceptible of growth at all, will have the most sure and rapid growth. Which grows most and in the best manner, the flower which is whole and perfect in its incipient state, or that which has a canker in it, or is otherwise injured and defective in some of its parts? Which will grow the most rapidly and symmetrically, the child which is perfect in its infancy, or one which is afflicted with some malformation? Illustrations and facts of this kind seem to make it clear that the spiritually renovated state of mind, which is variously called holiness, assurance of faith, perfect love, and sanctification, may be susceptible of growth or increase. It is not only evident that there is no natural or physical impossibility in it, but, as has been intimated, we may go farther, and lay it down as a general truth, that perfection in the nature of a thing is requisite to perfection in degree. And accordingly, although it is possible for a person who is partially holy to grow in holiness, a person who is entirely holy, although he may be assailed by unfavorable influences outwardly, will grow much more. The obstacles to growth in holiness will not only be much less in the latter case than in the former; but that inward vitality, which is necessary to the greatest expansion and progress, will possess a positive and effective power, unknown under other circumstances.
— edited from The Interior or Hidden Life (1844) Part 2, Chapter 15.