The angel Ezra revealed a sigil exists, that when drawn, will open a direct portal to Heaven. The portal is apparently in the image of Metatron's Cube, however it is unknown if Metatron or God made it in the first place.
In Reichenbach, we see the sigil drawn, but there was no portal. By this, it is implied that there is an incantation or ritual to open the portal. Fallen angels, who no longer have the ability to fly, used this door to return to Heaven under Metatron's leadership. Following Metatron's imprisonment, the gates remain closed, while the angels have reclaimed Heaven. After the events of Inside Man, the angels moved the portal somewhere else as revealed in Out of the Darkness, Into the Fire.
In The Future, it was revealed that a human passing through the portal would instantly kill them and return their matter back to the universe while their soul ascends to heaven.