A heart unsanctified, which is the same thing as a heart not united with God, is a heart which has become disordered both in its faith and in its attachments. Its desires, in consequence of its faith being wrongly placed, are separated from their true center; and, consequently, are either given to wrong objects, or, by being inordinate, exist in a wrong degree. The sanctification of the heart is its restoration from this wrong state. And this is done by a course the reverse of that which sin has previously prompted it to take, namely, by the substitution of a right faith for a wrong one; by taking the desires from wrong objects, and by suppressing all their inordinate action. But this is a process which is not ordinarily gone through without much suffering.
— from A Treatise on Divine Union (1851) Part 5, Chapter 7.