Yitro 

יתרו

Until the giving of the Torah, connection between the heavenly world and this earthly world was not possible. At the giving of the Torah, the borders opened – the heavenly descended below and the earthly ascended above. 

To what is this matter comparable? It is comparable to a king who issued a decree and said: The residents of Rome may not travel to Syria, and the residents of Syria may not travel to Rome. Similarly, when the Holy One, blessed be He, created the world, He issued a decree and said: “The heavens are the heavens of the Lord, while He has given the earth to the people” (Psalms 115:16). When He sought to give the Torah, He nullified the original decree, and said: The earthly will ascend to the heavenly, and the heavenly will descend to the earthly; and I am the Initiator, as it is stated: “The Lord descended upon Mount Sinai” (Exodus 19:20), and it is written: “To Moses, He said: Ascend to the Lord” (24:1).  <source>(<i>Shemot</i> <i>Rabba</i> 12) </source>