The essence of mysticism is that God, the divine, or the Buddha-nature (in the case of Vajrayana) is actually within us, and so our objective is to bring that hidden reality into full fruition or visibility.
When we have realized that the Buddha-nature within us is the same as the all-encompassing Buddha-nature of the whole universe, then we have attained a kind of all-in-one unity.
At that point, we have attained the highest level of consciousness, but we choose to remain in the world and help others as bodhisattvas.