Rav Volf of Strikov zy’a asked why we open the door for Eliyahu Hanavi by Shfoch Chomascha, at the end of the Seder. Why don’t we open it and invite him in at the beginning, when we invite everyone else in and say ‘anyone who is hungry, come and eat’?
He answers that this is meant as an important lesson in ruchnius. On the night of the Seder, the doors of Shomayim are open to all Jews, and everyone can acquire great spiritual things. When we say that anyone who is hungry can come in, we aren’t really inviting guests. What we really mean is that anyone who desires to become sanctified can get what he wants. However, one must open the door on his own to let the great spiritual influences in, as a sign that he is willing to do his part.
A similar idea is said in the name of the Saraf of Kotzk zy’a to explain the Gemara (Brachos 28A) that says that Rabban Gamliel placed a guard at the door of the bais medrash to keep out anyone ‘whose inside is not the same as his outside’. We may ask how the guard knew what was inside a person. How could he know a person’s inner thoughts? The answer is that the guard didn’t let anyone in at all. Those who gained entry were those who climbed in through the windows, which was a clear sign that they had a true inner desire to study Torah.