The design of a single or double doorway, with flanking columns, appeared early in Jewish funerary art, synagogue mosaics, and paintings, and on glass, lamps, and later in textiles and manuscripts. At first it signified a physical symbol of the concept of the heavenly abode and, later, came to represent the Torah Shrine and the destroyed Temple of Jerusalem. Together with the *menorah, snuff shovel, *etrog, *lulav, and *shofar, the portal is one of the most common Jewish symbols found from the first centuries of the Common Era. The meaning of the design goes back to the ancient Oriental symbol for the residence of the gods on high. The doorway represented and signified the entrance to the heavenly precincts. Gods were portrayed standing or sitting in the doorway while the sun god Shamash from second-millennium-b.c.e. Akkadian art was frequently shown rising in the eastern mountains from between open double doors. These were the "portals of the sky" from which Shamash called out to the world. In Egypt, too, gods made their appearance standing between pillars that symbolized the heavenly sky. Later, in pagan art the portal was formed into a cult niche (aedicula or naos) holding the god and indicating his divinity. The early Jews conceived of a portal of heaven opening onto the house of God (Gen. 28:17). The Temple of Solomon is spoken of as the earthly residence of the Divine; the "glory of the Lord" enters, as did the image of Shamash in the Mesopotamian world, through its East Gate (Ezek. 43:4–7). The twin pillars that flanked the Temple, called *Jachin and Boaz, are also found in pagan temples of the Palestinian period. Probably the visual device of the portal was adopted into Jewish art from the neighboring Canaanites and Phoenicians, among whom the portal enclosed and sanctified the cult image. The Jews, having no cult idol, substituted Jewish symbols between the columns of the doorway. This is seen in the Jewish catacombs in Rome on the Via Torlonia where the portal was made in the form of a miniature Roman temple. It is shown with open doors, exposing the ends of scrolls, thus indicating the holy nature of the Torah. Painted directly above the Torah niche on the wall of the third-century-c.e. synagogue at *Dura Europos is a classicized portal probably symbolizing Solomon's Temple. Two columns supporting an arched lintel on a lead coffin from the first- and second-century necropolis at *Bet She'arim in Israel enclosed the menorah, thereby signifying the sacred aspect of the candelabrum. Other sepulchers are ornamented with elaborate portals and stone doors that probably retain some of the symbolic value of the heavenly portal. The sixth-century-c.e. synagogues at Bet Alfa and Tiberias have mosaic representations of the pedimented portals surrounded by other Jewish symbols. When the portal was used in Jewish funerary art it probably represented not only the holiness of the tomb, but also the gates of heaven through which the deceased had passed.
B. Goldman, The Sacred Portal (1966), includes bibliography.
"Portal." Encyclopaedia Judaica. . Encyclopedia.com. (August 15, 2018). http://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/portal
"Portal." Encyclopaedia Judaica. . Retrieved August 15, 2018 from Encyclopedia.com: http://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/portal