The reason why the Lord experienced these two states, the state of being emptied out and the state of being glorified, is that no other method of achieving union could possibly exist. Only this method follows the divine design, and the divine design cannot be changed.
The divine design is that we arrange ourselves for receiving God and prepare ourselves as a vessel and a dwelling place where God can enter and live as if we were his own temple. We have to do this preparation by ourselves, yet we have to acknowledge that the preparation comes from God. This acknowledgment is needed because we do not feel the presence or the actions of God, even though God is in fact intimately present and brings about every good love and every true belief we have. This is the divine design we follow, and have to follow, to go from being earthly to being spiritual.
The Lord had to go through the same process to make his earthly human manifestation divine. This is why he prayed to the Father. This is why he did the Father’s will. This is why he attributed everything he did and said to the Father. This is why he said on the cross, “My God, my God, why have you abandoned me?” [Matthew 27:46; Mark 15:34]. In this state God appears to be absent.
After this state comes a second one, the state of being in a partnership with God. In this second state we do basically the same things, but now we do them with God. We no longer need to attribute to God everything good that we intend and do and everything true that we think and say in the same way as we used to, because now this acknowledgment is written on our heart. It is inside everything we do and everything we say.
In this same way, the Lord united himself to his Father and the Father united himself to the Lord. In a nutshell, the Lord glorified his human nature (meaning that he made it divine) in the same way that he regenerates us (meaning that he makes us spiritual).
The chapters on free choice [Sections 463–508], goodwill [Sections 392–462], faith [Sections 336–391], and reformation and regeneration [Sections 571–625] below will fully demonstrate three things: that all people who go from being earthly to being spiritual go through these two states; that they are brought to the second state through the first; and that this is how they move from this world to heaven. Here I will say only that in the first state, [also] called the state of being reformed, we have complete freedom to direct our actions with the faculty of reasoning that we have in our intellect. In the second state, the state of being regenerated, we have the same freedom but we intend, act, think, and speak with a new love and a new intelligence that come from the Lord.
In the first state our intellect plays the lead role while our will plays a supporting role. In the second state our will plays the leading role while our intellect plays a supporting role, although it is still the intellect that acts in connection with the will, not the will that acts through the intellect.
The same process applies to the joining of goodness and truth, the joining of goodwill and faith, and the joining of the inner and the outer self.
from True Christianity, Section 105