Warning: SPOILERS for Lucifer Season 5, Part 2.
God (Dennis Haysbert) retired in Lucifer season 5, part 2, and His departure from the universe means there's a new Almighty: Lucifer Morningstar (Tom Ellis) himself! It seems that the Devil becoming God's successor was His plan all along. God works in mysterious ways and sees the big picture because He's omniscient, so it's likely Lucifer ascending to Godhood was what He intended for the Devil from the very beginning.
Michael, the villain in Lucifer season 5, set the chain of events in motion by gaslighting God into thinking the Almighty was losing His powers. God finally came to Earth to end the brawl between Lucifer, Michael, and Amenadiel (D.B. Woodside) but He decided to stay and spend time with his sons. Believing His powers were still getting out of control, God even became mortal for a short time. Based on what God learned from shadowing Lucifer and meeting the Devil's friends while He was human, God decided that He wanted to retire. Later, during an "emotional round of golf", Amenadiel helped God realize that Michael was manipulating Him. But that still didn't deter God from his retirement because He had a plan.
What God ultimately realized is that He was both tired from running the universe since the dawn of time and, more importantly, He was lonely. God and his wife, the Goddess (Tricia Helfer), became estranged eons ago because she felt God spent all of His time on humanity and lacked the same care for their marriage. This caused a rift between them and God even banished the Goddess to Hell. But in Lucifer season 5, episode 14, "Nothing Lasts Forever", the Devil sent his sister Gabriel to bring the Goddess back from her universe to reunite with God. Lucifer expected his parents to return to Heaven and rule together but instead, God and the Goddess decided to live in her universe. This is God's "retirement plan" but it would leave Lucifer's universe without a God.
When God and the Goddess left the universe, He promised Lucifer and Amenadiel that "you'll sort it out". But of course, God knew that Michael was plotting to become the new God and that Lucifer also wanted 'the promotion' because the Devil believed becoming God would finally make him worthy of Chloe Decker's (Lauren German) love. God also knew that His other children, the archangels, would choose sides between the twins, Michael and Lucifer, who were vying to become the new God. Tragically, Remiel (Vanessa Vidotto) was killed by Michael for aligning with Lucifer and the archangels (and Hell's demons) fought each other in Michael and Lucifer's name. God, in His omniscience, must have known all of this would happen as well.
Ultimately, God's plan was for Lucifer to earn Godhood by sacrificing himself and finally declaring that he loves Chloe. Since he's the star and main protagonist of Lucifer, everything was building up to the Devil taking all the lessons he'd learned over the last five seasons to take over for his Dad. What was also incredibly important, however, is that before God left, He finally told Lucifer what the Devil wanted and needed to hear since he rebelled against Heaven: that Lucifer was right that God could have been a better father, that He was proud of Lucifer, and that He loved His son. But now that God's work is finally finished and He has retired, it's up to Lucifer to live up to being the kind of God the universe needs in Lucifer season 6.