The warnings in the Bible are real. John 10 says his sheep listen to him and follow him, and nothing can pull them out of his hand.
I’ve seen it with people who drift for a while, sometimes years, and then return more committed once they see what they almost lost.
I end up thinking about Peter. In Luke 22:31-32, Jesus knows Peter is going to deny him, and he prays that Peter’s faith won’t fail. Then he tells him to strengthen the others once he’s turned back.
So Peter gets a job to do after his failure, before he ever hits bottom.
For someone grieving a serious failure, it’s a concrete place to start. Bring it to Christ instead of assuming you’re too far gone to be restored.