At the end of Lucifer S03E17, Lucifer says the line,
I think I've made a horrible mistake
What was the possible mistake?
Lucifer and detective were trying to capture murdered targeting couples. At the end he see another detective talking to "his" detective and feel jealous.
Ale the lyrics to song that Skye Townsend sing goes
An unfamiliar fire’s been burning
And I don’t how to feel
I’m not great at love, but I’m open
And out of all of the prayers out there, mine was chosen
So in summary Lucy realize that
He fell in love with a human women.