The purity that St. John of the Cross writes about when he writes of union with God is not merely freedom from concupiscence. The saint writes of how the closer the soul comes to union with God the more impure the soul finds itself. One way of putting it could be that the soul begins to see itself more and more with the eyes of God, before whom no one is pure. That's right: in reality, which is God, no one is pure.
And what is the touching point by which the saint comes to mature purity? It is through the process of picking oneself back up, again and again. It does not matter if the saint is free from the sins that used to be a recurrence for him or her: the way the soul experiences and comes into mature purity of union with God is through picking oneself back up, again and again, right up to the point of death - where "picking oneself back up again" comes to be more like "entrusting oneself to God more and more".
This is true even when picking oneself back up, or entrusting oneself to God once more, has not been occasioned by any event of sin. For the soul never comes to some point in time in which it can say, "I am totally pure. I am done." Which is what the person claims when he believes in, and practices, a "purity" that he defines precisely in doing what he could not do before (or rather, doing exactly what he did before, with the exception that he gives himself exception from sin according to his subjective level of what he decides to call freedom from lust, which is whenever he decides to call it total arousal sent from heaven's song instead, and with the exception that now he can add pride and delusion to his list of sins and errors) thereby proving he is "redeemed" and thereby, in actual reality, debauching any true ethos of redemption.
If the person does not realize this, that person will at some point fall into despair, having thought himself totally pure - that is, thinking himself able to be cognizant of how objectively pure he is by his subjective experience of not feeling lust (whatever that may be, in whatever occasion of sin), which is his precious subjective version of "not lusting".
This is the fallacy which Christopher West's teaching of "mature purity" sets one up for: either despair or the further hardening of the soul in presumption.