Religion was something that had sprung up while Lucifer had been imprisoned. It was an interesting concept, somewhat like the angels in its organisation, but the part that interested Lucifer the most was how, for something so pure in theory, it had ended up being the tool for humanity's worst feature: their desire to murder each other.
Lucifer truly didn't understand humanity, or even the religions that worshipped his Father (and the ones that worshipped him even less so), but he had been somewhat intrigued by the Christian idea of a specific day, today, Michaelmas, that honoured his closest brothers (despite humanity being a little fuzzy on how many archangels there were).
Lucifer didn't know what had happened in Heaven after his death, but he was willing to bet, knowing his brothers, that only Gabriel would care about this: the kid always did have a peculiar affection for humans. Lucifer wished he understood it, because maybe, just maybe, they might not be in this mess if he did. If he had taken the time to listen to Gabriel rather than dismissing him out of hand to spend time with his pet projects instead; twisting Lilith and Alistair's souls into something unrecognisable, simultaneously more and less powerful than they had been; whispering to Abel, and being surprised by the integrity of Cain. Cain almost made him waver in his conviction that humanity was completely flawed, until he took to being Lucifer's knight like a duck to water.
He wondered where Gabriel was in all of this. The kid had idolised him, back in their heyday; learning everything he could, teaming up to play tricks on Michael and Raphael in an effort to make them and Father laugh. Lucifer missed Michael every day, the loss of his companionship a pain in the very core of his being that he would now never be rid of, the missing second half of his grace, but he missed Gabriel too. He hadn't laughed in millennia. Even if Gabriel wouldn't help him (and he wouldn't; Lucifer knew that), he would have appreciated the easy companionship of his cheeky little brother.
But he knew, deep down, that he would never feel the touch of his brothers' graces again, never have that balm to soothe his fears and doubts. He had sacrificed that, and laughter, to prove their Father wrong, to restore perfection to this beautiful creation. And he would do it, by any means necessary, even if that meant killing the person who meant the most to him in all creation.