Scripture tells you that the nations could have been cut off all at once from the land given to the children of Israel; yet God willed that it should only be little by little. We can think of any number of other things which we admit might have happened or may happen, though we can allege no instance of their occurrence. We should not then deny the possibility of human sinlessness, simply because there is no [sinless] person, save him who is not only man but very God, in whom we can show this perfection actually achieved.
Divine grace ever stirs up the will of human beings, not so as to protect and defend it in all things in such a way as to cause it not to fight by its own efforts against its spiritual adversaries, the victor over whom may set it down to God’s grace, and the vanquished to his own weakness, and thus learn that his hope is always not in his own courage but in the divine assistance and that he must ever fly to his Protector. And to prove this not by our own conjecture but by still clearer passages of holy Scripture let us consider what we read in Joshua the son of Nun: “The Lord,” it says, “left these nations and would not destroy them, that by them he might test Israel, whether they would keep the commandments of the Lord their God, and that they might learn to fight with their enemies.” And if we may illustrate the incomparable mercy of our Creator from something earthly, not as being equal in kindness but as an illustration of mercy: if a tender and anxious nurse carries an infant in her bosom for a long time in order sometime to teach it to walk, and first allows it to crawl, then supports it that by the aid of her right hand it may lean on its alternate steps, presently leaves it for a little and if she sees it tottering at all, catches hold of it and grabs at it when falling, when down picks it up, and either shields it from a fall or allows it to fall lightly, and sets it up again after a tumble, but when she has brought it up to boyhood or the strength of youth or early manhood, lays upon it some burdens or labors by which it may be not overwhelmed but exercised, and allows it to vie with those of its own age; how much more does the heavenly Father of all know whom to carry in the bosom of his grace, whom to train to virtue in his sight by the exercise of free will, and yet he helps him in his efforts, hears him when he calls, leaves him not when he seeks him, and sometimes snatches him from peril even without his knowing it.
Although all the nations in the surrounding country were subject to king David, still God did not allow the Jebusites to be destroyed entirely, as he himself says elsewhere: “I for my part will not clear away for them any more of the nations. Through them the Israelites were to be made to prove whether they would fear me.” For this reason the prophet said to David, “Go up, and build an altar to the Lord in the threshing floor of Araunah the Jebusite.” That pagan king represented the people of the Gentiles. Notice, brothers, that no place in the land of the Jews was found worthy for the altar of the Lord to be built; but in the land of the Gentiles a place is chosen where the angel is seen and the altar of the Lord is built, and thus the wrath of the almighty Lord is appeased. Then already was prefigured the fact that in the hearts of the Jews no worthy place could be found to offer spiritual victims; the land of the Gentiles, that is, the conscience of Christians, is chosen as the place for the Lord’s temple.
Some of his [humanity’s] petitions God grants him promptly (I mean those without which no one can be saved), but some He withholds from him. And on certain occasions He restrains and dispels from him the scorching assault of the enemy, while on others, He permits him to be tempted, that this trial may become to him a cause for drawing near to God (as I said before), and also that he may be instructed and have the experience of temptations. And such is the word of Scripture: “The Lord left many nations, without driving them out; neither delivered He them into the hands of Jesus [Joshua], the son of Navi, to chastise the sons of Israel by them, and that the tribes of the sons of Israel might be taught, and learn war.” For the righteous man who has no consciousness of his own weakness walks on a razor’s edge, and is never far from falling, nor from the ravening lion—I mean the demon of pride. And again, a man who does not know his own weakness falls short of humility; and he who falls short of this, also falls short of perfection; and he who falls short of perfection is forever held by dread, because his city is not founded on pillars of iron, neither upon lintels of brass, that is, humility.
[AD 430] Augustine of Hippo on Judges 2:20-23