WARNING: The following contains major spoilers for Lucifer Season 5B, streaming now on Netflix.
After four and a half seasons of the much-discussed but invisible God presence in Lucifer, he has suddenly become a major figure in the human body of flesh and blood, convincingly portrayed by (Dennis Haysbert). However, his motives for coming to Earth and interacting directly with his sons and various humans remained initially a mystery to both the audience and the characters. The true story turned out to be something no one expected - perhaps even God himself.
In the first few episodes after his appearance, God acts much like any visiting parent: cooing at his grandson, insisting on a family dinner, and making critical notes on everyone's life choices. Lucifer, frustrated by the turmoil of his personal affairs on top of his already strained relationship with his father, continues to question why God there and does not get a direct answer.
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Then things get weird. While the events of the "Bloody Celestial Karaoke Jam" - i.e. people who indulge in singing and dancing without realizing it - were initially explained as nothing more than God's pleasure, he later admitted to Lucifer that he wasn't sure he was really in control of his powers. It also causes (Daniel Espinosa) to explode (albeit instantly reverses it), spends a day floundering as a human and then forgets how to regain his omnipotence.
As events unfold, it is clear that they could lead to some sort of cosmic-shattering catastrophe, and God's decision to retire and pass the torch to one of his descendants before that happens appears to be the only responsible choice. Lucifer and Amenadiel learn the plan from their sister Gabriel, also known as the Gossip Angel. "My dad is growing up, and he's losing it," she says, continuing explaining that Lucifer's hostile twin brother Michael noticed it first but God's deceptions on Earth proved it.
Thanks to the first half of Lucifer Season 5, the audience already knows better than to trust anything Michael says or does. Sure enough, Amenadiel engages God in private conversation and proves that Michael was gassing his father, convincing him that he's unstable. This is based on a major theme from the show that gradually emerged; That is, angels are self-actualizing to take on physical features that reflect their self-perception. Amenadiel correctly concludes that God is the same way and that his doubts about his control over his power have in fact manifested.
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Although Lucifer was eagerly anticipating his father's retirement so that he could take up the position of God, he changed his mind when God announced that he would not only step down but leave the universe forever. Michael's manipulations are brought up and rejected, because God makes his decision based on what he wants to do for him and his wife, the Goddess, rather than his ability to continue to rule.
Just before the god and goddess left, Lucifer stopped them and asked another question: "How many of these was your plan?" God's only response is a perceptive smile, which could mean that his unspoken answer is "all of it," including his period of weakness on Earth and the eventual winner's battle between Lucifer and Michael. However, it can also mean that God intended his time to be spent as God only. Either way, Michael's plan fails, and it eventually leads to Lucifer taking the throne of heaven.