Man corresponds in his position, and may be said to be united with God in the work of his personal recovery, when he willingly and firmly yields his disfigured spirit to the restoring power of the hands of the great workman. In other words, he unites with God in his own restoration, when he lets the great Master of the mind work upon him.
There is an illustration of the subject to be found in the prophet Malachi: "Who may abide the day of his coming” says the prophet, "and who shall stand when he appeareth? for he is like a refiners fire and like fuller’s soap. And he shall sit as a refiner and purifier of silver; and he shall purify the sons of Levi, and purge them as gold and silver, that they may offer unto the Lord an offering of righteousness."
The great trouble with men is, even when they have some sense of religion, and begin to estimate its value, that they are unwilling to let the Spirit of God perform his appropriate work upon them. Sin has attached itself to the spirit's surface, like dross to the pure gold. Not more insinuating than it is adhesive, it intertwines itself with man's powers and mental exercises with indescribable strength; so much so that it is difficult to separate the good from the evil, to detach the pure from the impure. And it never can be done effectually and truly without the operations of that omniscient Spirit, which are "quick and powerful, and sharper than any two-edged sword, piercing even to the dividing asunder of soul and spirit." [Heb. 4: 12.]
We cooperate, therefore, with God, in the work of personal redemption, when we submit to this divine operation without reluctance; — willing to be placed in the crucible, and to be subjected to the fiercest flames till everything evil is consumed and taken away. This is what some ancient experimental writers call death, that is to say, death to nature, or rather to the corruptions of nature. Occasionally varying the expressions they employ, they sometimes call it crucifixion or inward crucifixion. As Christ died in the body, say these writers, so we must die in the spirit; — as Christ was crucified and laid in the tomb, so we, in the spiritual sense, must be crucified and be laid in the tomb with him. The expressions, though they may sound singularly to some, convey a great truth, which has a permanent foundation in the principles of morals and religion. We cannot be allied with God without freedom from sin. To be free from sin is obviously to die to sin. And it would not be easy to die to sin, without going through that process of inward crucifixion, which is the antecedent of death.
— edited from A Treatise on Divine Union (1851) Part 7, Chapter 3.