Fr. Sieg, «The Introductory Formula of Rev 4:1-2a in the Context of Traditions.», Vol. 18 (2005) 33-43
The formula of Rev 4:1-2a is composed of three phrases: 1. "Then, in my vision, I saw"; 2. "a door open in heaven"; 3. "the same voice... With that, the Spirit possessed me". The first and the second phrase, which serve to introduce a new vision, were most likely taken from the Pseudoepigrapha of the Old Testament, where they find plentiful analogies, as features of literary style. For the third phrase we cannot find precise parallels in other sources. The author of the article arrives at the conclusion that the subject of the voice (4:1c) was the "One who was sitting on the throne" himself, who, by means of his voice, introduces the visionary into a state of ecstasy.