The following is a letter of John Newton, which both shows the nature of progressive sanctification but more importantly it shows the absolute depth of our continual need of a savior (Letter 7 in the collected works vol. 1)
The complaints you make of what passes within, encourage me under what I feel myself. Indeed, if those, who, I have reason to believe, are more spiritual and humble than I am, did not give some testimony that they find their hearts made of the same materials as mine is, I should be sometimes hard put to it to believe that I have any part or lot in the matter, or any real knowledge of the life of faith.
But this concurrent testimony of many witnesses, confirms me, in what I think the Scripture plainly teaches, that the soil of human nature, though many spots are certainly better weeded, planted, and manured than others, is every where the same, universally bad; so bad that it cannot be worse, and of itself is only capable of producing noxious weeds, and nourishing venomous creatures.
We often see the effects of culture, skill, and expense will make a garden where all was desert before. When Jesus, the good husbandman, encloses a soil, and separates it from the waste of the world, to make it a residence for himself, a change presently takes place; it is planted and watered from above, and visited with beams infinitely more cheering and fertilizing than those of the material sun.
But its natural propensity to bring forth weeds still continues, and one half of his dispensations may be compared to a company of weeders, whom he sends forth into his garden to pluck up all which he has not planted with his own hand, and which, if left to grow, would quickly overpower and overtop the rest.
But, alas! the ground is so impregnated with evil seeds, and they shoot in such quick succession, that if this weeding work were not constantly repeated, all former labour would be lost. Hinc illæ lachrymæ. Hence arises the necessity of daily crosses and disappointments, daily changes of frame, and such multiplied convictions that we are nothing, and can do nothing, of ourselves; all are needful, and barely sufficient, to prevent our hearts from being overrun with pride, self-dependence, and security.