In the space of a few sentences in Proteus, Stephen manages to associate himself both with Christ ("Come. I thirst") and with Satan ("Clouding over. No black clouds anywhere, are there? Thunderstorm. Allbright he falls, proud lightning of the intellect, Lucifer, dico, qui nescit occasum"). In terms of his personal mythologizing, this makes sense: both figures appeal to Stephen, as they did to Joyce. There is also some connection in the name Lucifer, which can apply to both Christ and Satan.
In John 19, the Savior hangs on the cross: "After this, Jesus knowing that all things were now accomplished, that the scripture might be fulfilled, saith, I thirst. Now there was set a vessel full of vinegar, and they filled a spunge with vinegar, and put it upon hyssop, and put it to his mouth. When Jesus therefore had received the vinegar, he said, It is finished: and he bowed his head, and gave up the ghost" (28-30). The scripture which is being fulfilled is Psalm 69:21, "They gave me also gall for my meat; and in my thirst they gave me vinegar to drink." Stephen's thirst is almost certainly for alcoholic beverages, which he has promised to Mulligan but will instead enjoy with the men he meets in the newspaper office.
Tradition holds that, before he fell from heaven, Satan's name was Lucifer (in Latin, "light-bearer"). The name Lucifer has also been applied to the morning star—a bright planet, usually Venus, seen in the east before sunrise. In Isaiah 14, the decline of the morning star images the fall of an enemy (human?) of God: "How art thou fallen from heaven, O Lucifer, son of the morning! how art thou cut down to the ground, which didst weaken the nations! For thou hast said in thine heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north: I will ascend above the heights of the clouds; I will be like the most High. Yet thou shalt be brought down to hell, to the sides of the pit. They that see thee shall narrowly look upon thee, and consider thee, saying, Is this the man that made the earth to tremble, that did shake kingdoms; That made the world as a wilderness, and destroyed the cities thereof; that opened not the house of his prisoners?" (12-17). In Luke 10, Jesus refers to Satan falling from heaven not as the morning star but as lightning: "And he said unto them, I beheld Satan as lightning fall from heaven" (18).
But the morning star and the name Lucifer sometimes symbolize Christ. Stephen's Latin sentence means "Lucifer, I say, who knows no setting." Thornton notes that Father John P. Lahey identified the source of the phrase in a hymn sung by the deacon in praise of the paschal candle during the Easter week Holy Saturday service: "Flammus eius lucifer matutinus inveniat. Ille, inquam, lucifer qui nescit occasum" ("May the rising star of morning find it burning still—that morning star that knows no setting"). Gifford notes that, in the language of the 1962 Layman's Missal, this chant acclaims "the light of the risen Christ"—a star that knows no setting.
Gifford suggests that the light-bearer who died on the cross and the one who fell from heaven may also be linked through the "Thunderstorm" coming to Dublin. Luke's image of Satan falling from heaven like lightning informs Stephen's thought about the "proud lightning of the intellect." But Christ's death too was linked, if not with lightning, then at least with cataclysmic natural events: "And when the sixth hour was come, there was darkness over the whole land until the ninth hour. . . . And Jesus cried with a loud voice, and gave up the ghost. And the veil of the temple was rent in twain from the top to the bottom. And when the centurion, which stood over against him, saw that he so cried out, and gave up the ghost, he said, Truly this man was the Son of God" (Mark 15:33-39). This darkness, interpreted as an eclipse of the sun in Luke and linked with an earthquake in Mark, is never attributed to a thunderstorm in the gospels, but it seems possible that Stephen is making such a connection.