Moral union of the will exists when the will is united with God by means of moral enforcement merely, that is to say, under the constraints of moral obligation, without the consenting and affectionate concurrence of the heart. Such an union, which can exist only in respect to outward acts, makes what the world calls a moral man, but not a religious one. When a man does what God commands,— in other words. does what is right in action, but does it in opposition to his own selfish desires, — he is in union with God, if we may so express it, morally, or in the outward manner, but not affectionally, or in the inward disposition. He is a man divided; partly for God, and partly against him. His conscience is right, but his heart is wrong. In the language of the apostle Paul, he does that which he hates to do: he does good, but "evil is present with him."
Some would, perhaps, say, that a union so imperfect as this, including only a part of our nature, is not to be regarded as union in any proper sense of the term. But looking at the subject psychologically, that is to say, in reference to the nature of the mind, it is obviously a positive or real union as far as it goes. Undoubtedly it is imperfect. It has not that full and broad basis which it might have, and which it ought to have. But still it is something, and especially because it involves that conviction of mind which is likely to lead to something else better. He who observes the Sabbath, not because he loves to observe it, but because his conscience requires it is in a more favorable condition than he who has neither conscience nor love. But if something is done, it is still certain that the most important part remains to be done.
The union of the will, which has just been described, becomes consolidated and perfect when we add the concurrence of the affections to the supports of the moral sense. It is this union which we have denominated affectional. In order, therefore, to that union of the will with God which is requisite in the highest state of religious experience, the action of the will, in harmonizing with God's will, must rest upon the twofold basis of the approbation of the conscience and of the love of the heart. In any other state of the mind, the union of the will with God is more or less obstructed and enfeebled. When, in connection with the moral union, the obstruction of all discordant tendencies and desires is out of the way, and the affections are in the right direction, the union is such as it should be. Of a will thus united with God, it may be said, with almost literal truth, that it is the subject of a new creation, and has a new life.
— from A Treatise on Divine Union (1851) Part 5, Chapter 4.