I'm not a parent with trauma but I was a child who was never rescued from my abusive home.
I disagree with the idea that faith is the most important thing, but that could be because the trauma I experienced was religious. Religion often makes children of trauma feel gaslit because we're supposed to be healing through it but we don't feel any better so it makes us feel that something is wrong with us. This can even lead to more anger and resentment (because isn't god supposed to save us if we pray). It can also go the opposite direction with us being too trusting and falling into cults in a desperate attempt to find meaning and answers.
So it's much more important to try to build a trusting relationship with the child and help them learn how to develop a strong sense of self so they can think critically and not do things just for the sense of connection they lacked in childhood. The child might initially mistrust you so it's important to give them space and also never assume you know what happened. If they say something didn't happen, trust them.
My mom thinks I have repressed memories of being raped and refuses to believe I was only abused emotionally and physically. It's extremely damaging to not have the horror you went through be enough for adults around you.
I'd get family counseling as soon as possible, but never treat therapy like a punishment or something for crazy people. I waited way too long to get help because my stepdad would institutionalize me for standing up to him and say I was going to end up crazy like my mother. Mental healthcare is something everyone needs and if you're in it too it models that it's not something they have to do alone.
Also expect some emotional outbursts as they heal, especially if you get into EMDR therapy (which was extremely beneficial to me). Think of it as growing pains. Emotional abuse freezes us in our developmental stages so when we start growing again it's going to hurt. But the more we work at it, the easier it becomes.
It's also important not to coddle too much. They need to have some leeway and understanding but it's incredibly easy to grow personality disorders from childhood trauma. Abused children learn how to manipulate adults to survive. Make it clear that they don't need to do that anymore.
But remember that you're doing a good thing. Nobody would take me in or believe me when I was growing up so now I have trouble trusting people. You're providing a safe foundation and a place to come back to which is so important.
EDIT: forgot to say that hobbies are so important. I don't mean sports. I mean artistic hobbies, especially ones that have them socialize. Theatre saved my life, so did writing. It's important to give them that outlet. But let them direct the activity. If they say they're not ready then they'll resent it and it'll have the opposite effect. Never shame them for their interests even if you think they're a waste of time. And support groups with other teens like them can also be incredibly helpful.