What do you think of people who grew up Muslim but then rejected their faith?
It really depends on the reason they reject their faith. If it’s just because they didn’t want to put time into practicing it, or some asinine, lazy reason like that, then I think it’s really shameful that they would reject their faith. If it’s a matter of wanting to “fit in” or trying to avoid the harmful stereotypes associated with the faith, then it’s between them and Allah and I have no right to judge.
The way I see most Muslims reject their faith is simply because they don’t want to practice a religion (and I don’t mean a turn to Agnosticism or Atheism; I mean they just don’t want the responsibility of practicing a religion), or, most commonly, they identify as a “Muslim” but then do things like drink, hook up, do drugs, and just generally do haram things.
Those latter types of “Muslims”, particularly Muslim men, are the worst. They’ll drink and do drugs and have sex with tons of women, but when it comes down to settling down and getting married, they want only “pure” girls, who have kept their sobriety and their virginities. This double standard exists in Western society as well, but it’s especially toxic in the Muslim community because these men genuinely believe they deserve a religious girl simply because they never officially absolved themselves of the title “Muslim”.
But in all honesty, it is not my place to judge those who reject their faith. People do it for a variety of reasons, some better than others, and it is up to Allah to judge them for this decision. God is good and merciful and forgiving, alhamdullilah, and I can only hope that these people who abandon their religion find their way back, or at least find peace with themselves.