In the physical world, even if a door is closed, you can still see that there is a door there. In the spiritual world, however, you cannot see the door until it opens. The door itself is invisible: it’s a solid wall. A person can go through the door only when he corrects himself sufficiently.
The entrance into the spiritual world becomes revealed to a person as soon as he likens his qualities to those of the next degree. But he will not see the door or the opening even a second before that.
As soon as a person makes his qualities similar to the next degree by acquiring the quality of bestowal that is being demanded of him, then at that very instant he sees the door and it opens for him. Then the Kli and the Light unite.
(From Part 1 of the Daily Kabbalah Lesson)