When you see the first light of dawn, you don't mistake it for the sun. Nor do you mistake the morning star for the sun. Nor, if there is a crescent moon, do you mistake this for the Sun either.
Paradoxically, Christ is called the "morning star" in the Apocalypse of St. John,
"I Jesus have sent my angel to you with this testimony for the churches. I am the root and the offspring of David, the bright morning star." (Rev 22:28 RSV) But more commonly we know Lucifer as the "morning star" as it suggests in Isaiah "How you are fallen from heaven, O Day Star, son of Dawn! How you are cut down to the ground, you who laid the nations low!" (Isaiah
How do we resolve this paradox? This angel, whose name means light-bearer, fell. But if you compare the light of Venus in the morning to the light of the real light-brining, the Sun, there's no confusion at all. The light of the morning star Venus depends on the real morning star, namely, the Sun. If Ahriman (whom Steiner identifies with Mephistopheles) is a hungry selfish lazy darkness, Lucifer by contrast is a stirring to action, albeit action of dubious value. This is where Lucifer's appropriate name might be considered Rubifero, bearer of muddy light, not light itself.
The saying "Christus verus luciferus" means Christ is the true light-bearer, which overcomes the one-sided prejudices both of Lucifer and of Ahriman. Whatever light Lucifer has is a dim reflected light, as Venus has but a dim reflection of the Sun's light.