It implies, in the first place, that the love which thus unites them shall have the same origin. The two streams must flow from the same fountain. God's love is in and from himself. Man's love, in order to be in harmony with it, must be in and from God also. It is impossible that the pure or perfect love which "loves God with all the heart, and our neighbor as ourselves," should rest on any other than a divine and infinite basis. It is of a nature so high, flowing out freely and cheerfully even to those "who hate us and despitefully use us," that it requires and can accept nothing less than God for its author and supporter. This sentiment we have already expressed; but it is so important that it will bear repetition. Man has not strength enough to sustain himself in the exercise of pure love, breathing out, as it does, its aspirations of benevolence towards its enemies, except so far as he rests upon God, and becomes a "partaker of the divine nature."
The union of God and man in love implies, in the second place, that man's love must not only be from God so as to be nothing more or less than a stream from the everlasting fountain, but it must flow out without adulteration or modification — in other words, it must be like God's love.
And this love, as it exists in him now, which consists in a sincere desire for the happiness of all beings, simply because they have a being or existence susceptible of happiness, is now, and always will be, the original and basis of all other true love. It was this love, which, in the bosom of eternity, prompted the plan of salvation. We cannot experience the blessed state of perfect union with God in love, unless our hearts are filled with a love of this kind. Our love must not only have its origin in the divine nature, in God himself, but must be like his. So that it should be our constant prayer, that God would give us a love-nature, which, in being kindled from the eternal fire, will burn of itself; which will send out its divine blaze in the midst of persecutions; and which "many waters cannot quench."
— edited from A Treatise on Divine Union (1851) Part 4, Chapter 6.